It depends on a lot of factors. Breed, age of the bitch, how well she handled the first pregnancy and recovered afterward, etc. The general wisdom is a year between litters, but there are situations where more time is warranted and even some where back-to-back breeding is okay, provided the bitch gets a nice long break afterwards - though someone should be VERY experienced with breeding to make that call. Minimum one year break tends to be safer, from what I know. Three to four litters on one bitch is the average 'safe' number, but again, it can depend on the breed and the age the bitch starts reproducing.
Giant breeds take longer to physically mature and have shorter lifespans, for example, so even a very healthy bitch who's an excellent mother may only have two litters, because she might start at three and be considered a senior by five.
Disclaimer: I'm not a breeder, just a biology and dog nerd who's interested in the process and who also has done research when getting my youngest, well-bred purebred dog to make sure I was supporting an ethical and responsible breeder.