The most active part is the modding channel. Where Lexx and NovaRain are active daily.
Since Russia started the war, the politics channel is also a bit active. The project-chat channel has some activity from Kamaz from time to time about his
Space Wreck game, other people come by to talk about other things but Space Wreck is usually the most active topic.
The gaming and fallout-general channels see some activity from time to time too.
Mr Fish used to pretty much keep the NSFW channels active, but he hasn't been active for a while.
Otherwise is pretty inactive most of the time.
How many people are actually on the forum? I have no idea, but there are a few. These are the ones I know are on here too:
Me, Hassknecht, Lexx, NovaRain, .Pixote., Korin, Kamaz, Gizmojunk, Mr Fish, Squadcar, Valdetiosi, KingArthur, DarkCorp, william dempsey (I'm pretty sure he's the Discord user chestikov2011), Keyboard Gecko, Walpknut, zegh8578, Serifan, mobucks, Millim, The Dutch Ghost, phobos2077, naossano, lolpop109, Hardboiled Wanderer, Femic, Einhanderc7, TheHouseAlwaysWins.
Of course, most of these are as inactive (or even more) there as they are in here.