Yes, there is a cap. I do not know what this cap is, however. It's most definitely no higher than 150 due to Deej talking about it in a Podcast. My personal guess would be much lower, like a few dozen. Note that when you convert a group into a clan, your members do not become clan members. They will instead be switched to followers, and will have to each manually choose to join the group as a clan regardless of if they are currently a group member.