Quote from the Kennel Club site
The Kennel Club:
Before thinking about breeding from your bitch, you should acquaint yourself with the Kennel Club Rules and Regulations on registering litters and the registration system in general. You will need to remember that the Kennel Club will not accept an application to register a litter when:
a) The bitch has already whelped six litters, or
b) The bitch was under a year old at the date of mating, or
c) The bitch has already reached the age of eight years at the date of whelping.
So I guess the answer is the maximum is 6 litters whelped between 14/15 months (earliest) and 8 years (latest), that is if you want them to have KC registration.
TBF (and this is just a personal opinion) I would have thought that any caring breeder would wait till at least 18 months and leave at least one season minimum between each whelping and mating again.