It really doesn't matter - join as many as you want.
You'll find very quickly that a lot of clubs won't be what you thought they were, but the only way to really find out is the hard way by joining them and getting involved.
You'll also quickly discover how many clubs you can handle participating in before you get too overwhelmed yourself, but you'll need to participate in their events to find out. This won't even be a strict number, either - it depends entirely on how much stuff the club does, and when they choose to do it. For example, the debating society has debates every Monday - even though this is a weekly regular thing for a few hours, if you have Monday nights completely free, this might work really well for you. However, the biological society often has events during a lunch time - if you've always got a class, or do most of your work during this time, this club might not work for you - even though they don't run as many events as the debating society does.
At the end of the day, you won't disadvantage a club if you join, so don't feel like you'll make it difficult for them. This is entirely a personal thing, and something you'll really have to figure out for yourself.