Itt if there are only 15 active users at the same time, then you shouldn't worry and FreeFlarum can handle it.
And the 250 user limit can go higher than that, we host bigger forums that are fine with it
However, FreeFlarum is definitely not suited for 250 users active at the same time (that are eg.: posting articles, browsing the forum, etc...). On the other hand, we didn't test any extreme numbers yet, but I am pretty sure FreeFlarum wouldn't handle it.
Simply said, FreeFlarum can handle 15 active users, even more (again, not tested, but I guess ± 30?) and a fair amount of... just users (specifically one of the biggest forums we host has +500 users and +1000 discussions and works just fine - the actual limit is "average" of all forums. If more forums would hit this limit, it would be a problem).