Unable to build mining corps in enterprise

I'm running an enterprise on WG and after building a bunch of corps, I'm now unable to start any new mining corporations at all, the mining button doesn't appear at all on the starting a new corporation screen (all else are there).

Is there a limit or rule that is preventing me from opening up new mining corps?

Yes. Only "small enterprises" can build mining corps, meaning under 100 corps. But you can still purchase the shares/bid on mining corporations that are already built in order to bring them under your control.

To add to Stratis advice, some people have 2 or more enterprises to let them continue building mining corporations. One enterprise is the primary and the other is exclusively for building mining corporations that primary buys when they are old enough to be bid on.

looks like a 2nd enterprise coming lol, hate buying mining corps if not owned by c3 and they far and in between