I am a member of several groups and average over 150 deviations per day in my inbox but I do enjoy most of them. As for a limit I don't really care how many you send I will look at them all as I always have. I love fantasy art and enjoy seeing everyone's different ideas.
I was going to say 10, but I honestly don't care how many there are, the more the better. If I have time, I love to look through them all and if I don't, I get to them later, the point is to share and enjoy.
I have tons of deviations to take a look at but it doesn't matter as I like viewing them. I don't take a closer look at them all (like photo manipulations etc, as I'm more interested in paintings and drawings as I'm a painter and drawer myself), but I check out most of them. The other works don't mind me at all, though, but I just view the pictures I'm more interested in.
This group system is cool as you don't have to watch tons of people to see different kind of art as you see it trough group submissions.