I have 36 comics on my reading list here on Tapastic, and it has been growing very slowly over the months I've been here.
I tend to subscribe to only a few comics because a.) I'm very busy, b.) I'm very picky, and c.) I rarely go browsing with the intent of finding somehting new. I tend to stumble on stuff here on the forums, or be linked to stuff by friends, etc. Also, due to point b.), I sometimes go back and read a comic more than once before I decide to subscribe or not.
Also, I don't subscribe to a comic until I've read the existing content. This is to keep from adding comics to my list that I will later have to drop - either because I don't like the way the plot is going, or because the creator decides not to finish it, or whatever.
As for managing to keep up with the comics that are there... It helps to have as few as I have. It also helps that I've got a lot of practise reading stuff in parallell. When reading novels, for example, I usually have 2 or 3 going at the same time, and switch between them, without getting lost. It's an acquired skill, and it works well with webcomics - especially since webcomics tend to update slowly.