Plebbit is fully p2p, serverless, decentralized. So there’s no way of knowing how many subplebbits there are, as there’s no central server. You can create a sub and never share its address and nobody will find it (unless you post in it with an account and post to a known sub with the same account, so users will see the sub in your account post history). Having said that, we have a list of subs that we know of, it’s https://seedit.app/#/communities/vote. To add a sub, see: https://github.com/plebbit/temporary-default-subplebbits