Sadly, the online player counts are not what they used to be, not that they were ever particularly high.  The best times to play (when you're liable to find more people) are between about 4 pm - 11 pm U.S. Eastern Standard Time.  Also, the player base is divided between two different servers, one for Sins/Entrenchment and one for Diplomacy.  (So even though 90 people may be playing it online, you would only see 45 of them on the Sins/Entrenchment server.  Also note that people who are currently playing in games will be counted as being online but their names won't show up in the Lobby chat room channel.)  Note that most of the larger games, the 4v4s and 5v5s are played on Diplomacy, but you can still find 2v2s and 3v3s for regular Sins.  (It's easier to fill regular Sins games than Entrenchment since everyone has regular Sins.)  The good news is that most of the people playing regular Sins are newer players.  Most of the experienced pros who still play have Diplomacy.

By the way, to learn more about how to play the game, I suggest reading the Strategy forum.  Definitely read Raging Amish's unit discussion guide, which is a sticky post there.