Very tricky question!How often can you safely breed a bitch.
There simply is no such thing as "safe". So long as there is life, there are risks and danger.
I have looked for studies done on this and have not been able to find them. Historically, many bitches were bred on their first heat cycle, then every heat cycle after that and there are still some advocates for this.. with the idea being that NOT getting pregnant on a heat cycle is an abnormal circumstance for a bitch.
Bitches first heat cycles don't always happen at the same age. I have known bitches who have their first heat cycle at 6 months, and bitches who have their first heat cycle at 14 months.
Not all bitches have a heat cycle every 6 months. Some it's every 8 months, or even once a year! It can vary from bitch to bitch.. typically.. looking at the pattern of heat cycles of the dam/granddam will give some idea of the regularity of heat cycles.
Most breeder's won't breed a bitch until she is over two, to allow for some maturity as well as so that testing such as hip xrays can be completed.
Some breeder's prefer to wait until a bitch is four or five years old before breeding them the first time. It gives time for testing, to complete titles, a bit of time to allow for heath issues to rear their heads.. as well as to watch the parents/grandparents and other related dogs for health issues.
Some breeder's prefer to breed a bitch once at age 2 (or 3), then wait until age 5 or 6 to breed a second and last litter.
Most breeder's do not breed every heat cycle.. in most cases, it's not advantageous to the breed and that's a whole lot of puppies.
Breeding.. is kinda of like being a parent, there are different "parenting styles" and there isn't "one size fits all". Each breeder has individual reasons and rationales for what they do and why they do it. It may be that different dogs and different dogs breeds require a bit different approach too.
There are people who are going to tell you "You should.." do this, do that. in regards to breeding. In most cases, those are opinions, not fact/research based. Some people will say that it's "good judgement".. however different breeder's frequently have different opinions on "good".
Long winded answer, I didn't see anyone else answering.. and the reason why is it's not a simple question.
There is no such thing as a safe breeding. Just being alive is dangerous. Poodles can express SA, Addison's or bloat.. all without ever having been bred. Life.. is dangerous that is one of the reasons that it is so precious.