You'll get about 10 people in a multiplayer server at most, if you're lucky. It's fun to play for a bit but it's not very well optimized and is still in alpha.
It's an offshoot of Space Engineers, which just went into Beta a month or two ago. I have over 1000 hours in Space Engineers and 76 in Medieval Engineers.
If you're interested in it, give it a shot. Watch some videos on it first before you go in expecting CoE type of things though.