I wouldn’t get too worried about the Alliance system. All it does is apply friendly fire damage modifiers. This is really only useful in PVE where you are fighting things with health pools that warrant multiple attackers.
Two clans of 10 attacking individuals seems a bit weird and excessive. Unless friendly fire damage setting is set to 0%, they would damage each other more by sheer mass than their intended target.
With that logic why not drop it to 1 or 2? In reality dropping clan size Hurts solo players. Solo players are solo. Clanned players are already groups. The clan size doesn’t lower the number in the group. The group was formed long before they even created their characters, some even years or decades ago.
They simply break up into multiple clans and operate autonomously in conjunction. Let me tell you, a group of 4 clans of 5 is way worse than a clan of 20. Especially if you don’t know the 4 clans are friends out of game. That’s a bigger advantage than not.
Its actually better for the solo and small group of players to have larger clan sizes for that reason. And when I say larger clan sizes I mean not only setting the limits higher (or unlimited) but also give incentives to larger clan sizes.
A legit argument could be made to set it to 20. The idea is to mitigate the chances of overloading them. It would spread out the population a bit which would reduce the overall load.