plenty you can usually find matches in about 5 to 10 minutes
lag it exists its annoying not much you can do about it aside from try ur best
glitches also exist usually not to big a deal but sometimes very anoying like just half an hour ago some guy mutes me stuns me i deserved it for being an idiot but when i got up i was still muted i couldn't even kill my next target who was on top of a roof so i got stunned again then died then got my abilities back now although this sucks its only happened to me once as in ever which was today not long ago
whatch videos online for tips try and be stealthy patience helps a lot good luck.
If you count from the worldwide leaderboard there are around 1 million people ranked. But there may be some who have stopped playing or maybe have double account. However the number of people playing it right now is still plenty and it's easy to find a match if you have good connection and router (if you use one).
Lag is pretty much there sometimes and there is nothing you can do about it..but not so bad that it will stop you from playing. Then again it depends on your connectivity. Glitch is also there but major part of it have been fixed by the latest update from Ubisoft I think. At least for me, I haven't encountered bothersome glitch ever since the latest update.
Buy it and play it online! I'm sure you'll find yourself addicted. I've been playing it since December 2010 and am still on the run!