There are two schools of thought on this. One approach is to register with as many good agencies as you can find. Jobs are often placed with several agencies, but many are given to one exclusively and you don’t want to risk missing the right opportunity.
The other strategy is to go for quality over quantity and select no more than three or four agencies that specialise in your area of work and locality. Being on the books of lots of agencies might increase your chances of getting a great job, but it could just as easily see you inundated with unsuitable vacancies from commission-hungry consultants that are ultimately a waste of your time.
Try registering with a few to get a feel for what working with a recruitment agency involves. Then, if the offers aren’t coming the way you want, you can always sign up to more.