On all the various mods and variations, I don't doubt it's thousands. Minecraft took in all this generation's version of quake and half-life modders. In the late 90's early 2000's I remember tracking 20-30 different quake and half-life mods, most having small teams of people working on them. Minecraft is a couple orders of magnitude more popular, so I wouldn't doubt a few thousand. There's enough people making content for it that you can probably find a youtuber playing some new mod or specialty level with complex logic every day if you look.