A world with limited collective ability

I need to design a world where the abilities of individuals will not increase when they form a group. For example, a group of 100 people will not have a clear advantage against one person.

I'm mainly concerned with combat abilities, but it would be better if other abilities were included (such as production and research, etc.). My goal is that everyone can fight against any group of any size.

I have thought of some possible settings, such as everyone can cause unacceptable damage to any group (imagine a world where everyone can launch a nuclear bomb), but I don't want to use such settings. Are there other options? I accept any setting, including magic.

a world where the abilities of individuals will not increase when they form a group

Starting from that premise and drawing the consequences, it's easy to conclude that your world as described cannot host any life form more complex than something on an unicellular organism level, because what's the point of clumping together if there is no group advantage? Just making access to nutrient and environment more difficult?

I even doubt life could form at all if there is no advantage in creating groups.

Badly Led or Commanded

Groupthink is a term used to describe communal decisions made without thought of long term consequences or even longevity of the group, but are instead made to uphold artificial group dynamics. For instance, when people group together, there is some discussions that need to be made:

This dynamic becomes especially evident in combat situations where individuals are repressed more than they are given authority or decentralised command. Subordinates start lying about their successes to their superiors. More resources are expended in preserving or enforcing group dynamics then is used in being an effective collective force. In such a way, there are many examples of large armies being defeated by small ones.

Badly equipped or untrained

Short swords were developed to allow close-quarter combat, working as a group. In your case, if your individuals were wielding long swords, or stand-off weapons, it would actually be better if they were further apart acting separately. Similarly, some weapons or combat skills are more effective if they scout, roam or operate independently.

Operating as a group requires specialist training and equipment in order to ensure the sum of the group is more than its parts.

Terrain does not support large groups

It is commonly held that large groups in Urban combat or forests are not as effective in some cases as small groups (or even individuals).

There may be numerous obstacles preventing communication, or group manoeuvres. There may be terrain advantages to smaller or individual groups that are more adept and can react more quickly, such as limited information, bad intelligence, or situations that require adaptive thinking.

In summary, there are numerous examples where large groups are at a distinct disadvantage to individuals or small squads. Whether in command and control, or in factors that favour individual action over collective action, these have enabled large armies and nations to be defeated throughout history by smaller or more adaptive forces.

It is called "Conservation of Ninjutsu". A common storytelling trope that make supposedly elite wariors weaker individually based on how many of them are on screen. So a single ninja is deadly threat to main character but 20 such ninjas instead of winning without problems are defeated easily. link to TvTropes article on this topic

Nothing to it.

The [magical adjective] [magical adjective] [magical noun] of all [magical adjective] warriors channel [magical word-phrase] to allow their [violence-related word] abilities to become greater the more foes are arrayed against them, and it's been that way ever since the [setting-important magical beings] brought forth [important magical setting element] in the [plot-relevant historical event].

Fill in the slots with whatever you like.

The sorcerous dragon crystals of all psionic warriors channel the essence of the Arch-Fey to allow their martial arts abilities to become greater the more foes arrayed against them, ever since the Leprechaun Illuminati brought forth the School of Witchcraft and Wizardry in the cataclysm of 2004.

In your world, everyone is telepathic with no filter on thought transmission and little ability to screen out the familiar thoughts around them, with the effect getting worse the more familiar you are with people. The more people you gather together on a task, the more everyone is distracted by a constant stream of Bob worrying about whether he remembered to quench the fire and hearth, Sally imagining what she can do with Joe's haunches, etc. You would think that this would also heavily disadvantage the single fighter, dealing with a hundred intrusive thoughts, but in most such fights, that single fighter is not familiar with those attacking them, and therefore it is easier to resist.

This has had a detrimental effect on group efforts in general, but the culture has developed a tradition of working on tasks serially, completing a step and then leaving written notes for the next person.

The social effects are a bit strange. Families are usually less affected because an isolated couple generally have little trouble with each other and children grow up constantly bombarded by the thoughts of their parents and siblings, which molds their own thought patterns to conform until they're old enough to form their own positions and decide to leave the family for more likeminded individuals. Larger social gatherings tend to focus on mindless fun and often involve mind-altering chemicals to socially enforce that.

Your people are intensely adverse to sociality

Sometimes a rule of your world shouldn't have an explanation. This is one of them because no reader will ever relate to the reason you give. Nevertheless, let's throw a non-magical reason out there that defies common sense.

The inhabitants of your world hate getting together.

Evolution tends to be naturally competitive, the form this took with your inhabitants was competitive isolationism. The only time they care one whit about their neighbor is when their neighbor is close enough to cause their ears to itch. Then they become territorially violent.

One member of your society is greater than the whole of the society combined because if you ever got them in the same room together, they'd be so busy fighting to get out of the room they could never bring their attention to bear on just one being.

When you're in a fight, generally people go one at a time. It is of course a bit of a stretch, but there is a big core of truth in it. Say your buddy is making a strike towards an opponent, and you want to join in. Do you strike while your buddy is striking? Or do you wait until his attack has been 'resolved', the uncertainty settled so you can strike. Mostly it is the latter. The fear of striking your own is powerful.

We can apply an overzealous psychology to your people. Whether it is with guns or physical, you strike alone. The imagined threat of harming your own is so high that people simply not do it. You might think that a person vs thirty is still unfair. The person might be exhausted from fighting 29 other people. However, the mental stress of watching your buddies fight one by one can exhaust them physically, making it a fair fight.

This kind of world would change how war is fought. A tank or plane with bombs would still be feasible, but they would look very different. They would prioritise ways to attack one person and one only. Guns and explosives would not be used for fear of somehow hitting your own, or only in specialised conditions where they are certain they can't harm others. A one on one world, where each 'participating' feels the strain of all those who fought individually before them.

The people in your world draw on an energy field when doing things. When they fight, they pull energy from the field to power their attacks. The same thing applies to both labor-intensive tasks. This energy field can only provide a certain amount of energy to a specific area, and a person has no limit to the amount they can draw. This energy field would be something that provides energy individually to each of your factions. For example, in a fantasy setting the Human faction and the Elf faction would pull from different reservoirs.

While this does not completely solve the issue, it does help reduce it, and would pair well with some of the other answers. It also does not apply to non-physically-intensive labor or other similar situations.