> is there any statistics about [...] # of people who paid
> the license fee?

Number of licences:

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=5875&forum=14&start=100#69534

Some users have serveral licences though, so user number is lower.

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=6778&forum=3