Wlork And for the number of discussions, messages, what is the max limit today ?
Every 6 months (or when required), I run a script that clears inactive and too active forums. So, the limit is not really enforced (this means, if your forum hits the limits, it will still function normally). If a forum is "inactive", FreeFlarum will archive it (forum owner can reach out to me and I will reactivate it for him - we keep archived copies for 6 more months, if storage space allows it). In the case of "too active" forums ("forums that have more than 20 000 offline users, 20 000 discussions or 35 000 posts"), I will reach out to forum owners personally via their admin account's E-mail address and ask them to migrate elsewhere.
To be honest, the "Fair Use Policy" is more of an "be lenient with the server resources usage, so that I can afford the hosting and others can host their forums for free" type of thing, it is not something that is enforced entirely at the moment.
Wlork And if I decide to start with FreeFlarum and migrate to Flarum later, it'll be easy to transfer the data?
Yes - full database dump and assets directory can be exported at anytime from https://freeflarum.com/settings/account_settings ("Download your data"). I've already successfully migrated a few forums away from FreeFlarum in my spare time, should you need a help hand in the future