I joined boxing because it's good fun and had the best social life, well equal to rugby. Dance and cheerleading were the most popular for girls and too, had the better social aspects.
You can join as many as you have time for, and whilst they're intended as a long term thing, some people do drop societies as time goes on. Others are heavily involved throughout their degree.
Before I went to uni, my friend advised me to join two societies and go to them religiously. Didn't exactly listen, joined 3 and only actually been to 1 properly - but going to every single event from that soc was the best decision I made all yet. Made so many friends and had a great time!
I joined the Modern Languages, Dance and Anglo-Japanese societies. AJS was the one I stuck with, MLS didn't really offer anything fun and Dance was too cliquey for my liking. Just pick societies that you think would be interesting,maybe try some taster sessions during Fresher's? The athletic union societies seem to be very drinking based but others aren't as much if it's not something you're interested in.
Just join societies that interest you. I'm not saying you will, but there's no point joining some (that you aren't really bothered about) because you feel you have to.
Join anything you have an interest in and go to the first few sessions/socials!
I started out by going to 6 and managed to narrow it down to 2, I now go to just them and love it! But they aren't things I would have thought about joining initially. Just picked up a leaflet and went :P
I'm not too sure what societies my university do; I think they do a history society so I may join that as I love history and its part of my foundation year! I'll have a look during freshers fortnight