For sake of looking it up the most reliable information comes from Steam user counts.
issues with this are:
1) It only goes back to 2013
2) Players owned this external of Steam either from original box or sources like GoG or others
3) This only shows peak users online at the same time, not actual player base
Currently Baldur's Gate 2 has:
473 playing
907 in the last 24-hour peak
2,644 all-time peak
This shows a moderate player base of 300-700 over the years since 2013 up to today.
With your response about mod support, that just makes me ask a question. What in the game is modified? PHB rules?
Can't speak to the Steam numbers, but lots of different editions of the game have been released over the years and on lots of platforms. It's obvious that both WotC and Larian both considered BG3 the holy grail, so it's not like BG is some obscure, unpopular series. Otherwise they would have given it a new name. Early sales (whether in games, movies, or books) are often reflective of brand recognition, and Larian got a lot more early sales than they expected. Baldur's Gate is a very recognizable brand in the RPG community.
As for the mods, nearly everything imaginable. Whole new NPC companions with quests, voice sets, romances, etc... LOTS of those. New zones, whole new storylines. Some of the mods rework the rules or add new classes. Many change game balance or add new equipment. The old games have lots of enthusiasm so the modders are always coming up with ways to add new things or improve the games. Look up the Gibberlings 3 site for an example of the many mods.
