Often said that you can't have too many cramps. In a pro shop, I can see the logic; you may have multiple items to glue up against a strict time constraint. In the amateur shop, not so sure. Good cramps cost good money, don't get used all that often, and take up storage space. Better a carefully thought out selection of really good quality cramps, backed up with a willingness to improvise for one-off unusual jobs.
Have a careful think about the work you intend to do, and the cramps that will suit it. No point buying top quality 48" sash cramps if you intend to make jewellery boxes, for example. If you have a lot of carcase furniture to make up from solid hardwoods, then a set of four be a sound investment, price notwithstanding.
Thinking about the sort of general cramping duties that crop up other than glue-ups - work holding, 'third hand' duties and generally immobilising things while you carry out some task or other - the cramps I find most useful are a set of 4 6" F-cramps, followed by some 3" G-Cramps. I'm not a fan of big G-Cramps for woodworking; they're heavy, cumbersome and blooming expensive for anything of reasonable quality, and if it needs that much cramping force there's probably a better way of doing it, but the little ones (3" and 4" or so) are handy.
It's also worth bearing in mind that you can do a lot with some strips of scrap wood and a few wedges. A strip of 2" x 1" with a block screwed to each end to span the job with enough length extra to accommodate a pressure block and a wedge makes a cheap, recyclable sash cramp. A handful of longish bolts and a few nuts and large washers (raid the B&Q hardware aisle) plus a few hardwood scraps makes any design of small cramp a job might need, and the bolts and nuts will last pretty well for ever.
Other than that, keep an eye out for discounts on good brand names. Buy sets of four if you can, or at least pairs. One cramp on it's own is rarely much use, you always seem to need them in multiples when you do need them. Be prepared to pay good money for good quality, buy sets sparingly only when experience demonstrates that you need them and will need them again, and you should - with a bit of luck - find that happy medium of having 'enough' cramps without ending up with a storage nightmare or too much damage to the wallet!