I think part of that is going to depend on your dog, some dogs can handle the whole process like champs, others can have difficulties, sometimes extremely bad difficulties, which I'm sure you've heard of! Just understand that since your dog is smaller and brachycephalic that could lead to more issues (I know shih tzus can get 'pinched noses' and in all dogs look out for hernias, cleft palates and any midline deformities)
I think how many times you breed her should depend 1. how many heats she normally has in a year 2. how she handles breeding (has she been bred before?) 3. her age and health (knowing her parents and even siblings health can be useful as well) 4. your life around that time when she'd be due. Back when my mom bred dogs I remember it varied per dog, I do know she never bred back to back heats though, I can't speak for all breeders and their view on back to back breeding but she never did, it was a few year back though (she's stopped breeding now).
When I was younger we had a litter of puppies due the week before county fair (my county fair runs for 2 weeks straight, of which I have always participated in both halves, all 14 days of it) and that, in hindsight. was a bad idea. Days cut short, many trips home checking on them, I had to catch rides some days, stress about how they were doing, etc. TLDR; make sure you plan
ahead when she's in heat and see if that time works for you, and once she's bred, don't plan anything for those weeks after (and a while before) she's due. It'll just make things easier in my opinion

Good luck!