The impressive part is how big modding is in minecraft despite Mojang never providing a mod interface, I don't know it may be different now but the general route for a long time was decompile the java byte code make your mod and recompile. There were a number of heroic teams that made mod interfaces out of this but a whole lot of nothing from the actual devs. I came off the quake modding scene and was a bit horrified by the whole situation, with quake they gave you the game code, map compilers and later the source to the engine.

> how many current persons working on Minecraft are also Modders?

Probably less than you'd think: lots of people recognise they need to leave their work behind at work and not get too emotionally invested in something they love, because at work they'll be exposed to the gut-wrenching business-decision sausage factory.

> how many current persons working on Minecraft are also Modders ?

Beyond small toy mods id be truly shocked if anyone really was able to muster the strength to go right back to working on product after a long day of working on product, especially in game dev. With it being such a passion driven industry already, too.

I know some of the original Bukkit devs moved on to work at Mojang