Good to see some anecdotal evidence that people have found us here. From an admin perspective, there's not much of a way to get insight in to this. As you pointed out, the ~300 subscribers is a good indication that there are users from other instances subscribed here. There's no easy way to tell how many other instances have at least one subscriber, but for every instance that does have at least one subscriber, posts from this community could show up in the "All" feed of anyone on the instance, so that has some potential to expand the reach somewhat as well.