Hello. I am wondering, how can you see how many active users an instance has?

This website has that info and quite a bit more.

Second - if the worst happened, and you woke up one day, and the instance you created your account on shut down and disappeared on you, is that it for your account and profile? You’d lose everything and have to start over on another instance?

As things stand, yup it would all be gone. There is talk of work to make accounts able to be migrated. For now, create another account on another instance as a backup. Obviously it won't have your posts, comments, votes, etc. But you can at least make it moderator of any communities you moderate so you don't get messed up in that aspect.

Last question - what’s the best community to ask these types of lemmy newb questions on?

There are several good ones, including this one, Ask Lemmy, and a few others.