With socs, it depends largely on if there is an end product - if you join a musical theatre or big band soc, and there is going to be a production/show - then you can't really dip in and out. With the more social or esoteric socs, then it could be as many as you like. I see no reason why you couldn't be in Japanese soc, Christian Union, Economics, Comic books and so on all at the same time.
With sports, it depends how good you are and how good the team is. If you are a decent team sports player - rugby, hockey, netball and in teams maybe 1st to 4th - I can see that they will be quite demanding on you and I really don't see how you'd have time for another sport other than maybe a casual game of badminton on Sunday evening. Certainly if you're in a top competitive sport, I think you'd be expected to be at training several days a week and for things like rowing, you'd be expected to be in the gym almost daily if not daily. Exeter is a top sporting uni, and if you are good at a sport then there will be certain expectations on your time.
If you want to play casually, or play a non-serious sport, then the situation would be different. There are going to be intramural teams, development teams for the big sports and things like Korfball or Ultimate are never going to be on the same level as the others.