There is no clan mechanic in AG2, so no such rule can be implemented. If you were talking about AG3, the thread belongs here.
I do not agree that a user should be limited in how many clans they can join. Instead, I think a better way to prevent "ghost" clan members is to kick a user from a clan if that user does not do anything for the clan for 1 month.
Finally, further discussion of AG3 clans goes here.
If you were talking about AG2 clans, I will simply say this:
Clan threads are not allowed, so they are only allowed in profiles. Due to the lack of an official clan mechanic in AG2, there is no limit to the size of a clan or the number of clans a user can be in.
No point in having such a senseless rule; why would the number of clan memberships affect anything? Or more importantly, why are clans themselves even important enough to warrant more rules in AG2? Zilch.
Reading the thread's OP clearly suggests that he's talking about the current clans here on AG2 and with that, the number of clans some users have joined.
And talking about the amount of clans one can join is purely up to that user in my opinion... if someone wants to join 100 clans, I say let him/her do as he/she pleases! What's the harm and why should anyone have a problem with that? =)