At peak times...North American evenings and weekends, it is around 150-200 people. It is pretty easy to find a game during these times. It is pretty dead in the middle of the night though, and can take a while.
It can be a little harder to find an equally skilled opponent. There are still new players trying online play, but there are also a lot of skilled players. If you can get some games in with the skilled players, watch the replays after the game to see how they play. You will learn fast while you take your lumps.
I think the main reason the online community is smaller is the length of games. A regular Sins game takes about 1-2 hours, depending on the size of the map and number of players. An Entrenchment game can take about 1-3 hours, though occasionally I have had some that went 4 or 5 hours because all the new defensive options make it extremely difficult to crush an opponent who has fortified his position.
Some people play with friends and save the game and resume play at a later time, or play large games cooperatively online. While there is a dedicated multiplayer community, a lot of people like to play this game on epic size maps against many computer opponents offline, and that seems to be the direction the game went with the current expansion.
Sins is a pretty awesome game, but if you only like shorter multiplayer games that only take 30 minutes, you might want to wait for Starcraft.
