Since most people do not play in large clans but mostly duo or trio (very popular right now) I would even go so far to only allow 5 people in a clan. That is more then enough.

If 2 clans of 5 have an alliance with each other then you get the same number of people you would have now (10) when they where in a clan together. But it would be much harder for a clan of 5 to become the permanent alpha on a server since the alliance is only a temporary state.

Clans of 10 have it way too easy on official servers. While small clans always have a hard time beeing able to defend themselfs. Most private pvp servers have a limit that is way under official and that is for a reason.