There is no explicit limit on the number of wikis you can create/be admin of. However, Fandom would prefer that you spend time building each wiki up rather than mass create wikis. This is one of the reasons for the 90 day cooldown period for adoption wikis. I believe there isn't any such cooldown on wiki creations but just keep that in mind.
There might not be a fixed number of wikis you can create, but there are several measures that prevent you from creating them. As I remember, you can only create 2 or 3 wikis a day, and I believe staff have deleted over 100 wikis of someone who created them all in a few months. As Andrew mentioned, creating a wiki eliminates you from adopting any wiki in the next few months, and creating a new wiki every few days increases the chances that they will be closed (duplicate wikis, for example)