A discussion, complete with charts (or are they graphs…) over on NewTeeVee trying to get a handle on how many episodes a web series should have – both in total, and in terms of a “season.”
The idea of a season is very old-media, I think. There are plenty of examples of TV that is not in season format – talk shows, soap opera, etc – and there is no reason web series should try to force seasons into the mix online. Instead, it seems to me that you need to launch with enough episodes to attract an audience (though how many is a good question) and then you keep it going for as long as there is mutual interest from the makers and the viewers. Some ideas naturally lend themselves to a longer lifespan while others are around just long enough for us to miss them when they are gone.
Some of the best BBC series (The Office, Spaced) were quite limited in their run but they each made significant impact in the world.
What is most important is to make sure you have something worth watching to begin with and then if people like it try to keep up.