In reply to shumidrives:
How many... Depends how generous someone is feeling since they're presumably a gift. They're often a fair bit cheaper bought in sets of 3 or more.
Which ones to go for... If they're for sale in uk shops they'll be good and safe. The best choice of design for small, medium and large cams may not be the same so don't be afraid to go for more than one design as you build your rack, try before you buy and buy what you like. Where possible I'd suggest going for British made cams, support British business and there's no compromise on performance or quality, they're right up there at the moment.
What size... Mid size. Cams for the features you're climbing at the moment or will be soon. Generally that's finger and hand size (adjust for your local rock type and route preference), maybe fists. Wild Country 1, 2 and 3 is a pretty good staring point or the similar 'half' size set, whichever you prefer (Size comparison charts exist for comparing to DMM, BD, Metolius etc).
Options... There's nothing fundamentally wrong with 4CU or other dual stem cams (I have several 3CU and they're great) if you're on a budget, they're nicely made and work as well as others in the vast majority of placements. They can be a little more prone to kinking in a fall so if you hammer them they may not last as well as others. That said, I doubt the majority of cams ever see a fall! These days in a world of price matching and special offers they may not be the bargain they once were though. Shop around but bare in mind climbing shops with a physical presence in climbing areas are a valuable resource, support them when you can.
jk