Breeding is not without risk. While most litters are raised without any problem at all, I have lost a few does to problems specifically related to pregnancy or nursing. Of course, I have lost rabbits to many causes that had nothing to do with breeding, too!
As to how many times to try with a doe, I would ask, how badly do you want babies from her? Someone who is breeding for meat wouldn't want to waste time on an unproductive animal, whereas someone who is breeding a pet or for show rabbits might be keeping her in any case, so it isn't as big a deal if she doesn't "pull her weight."If dead bunnies are just a disappointment to you, I'd say, let her try until she gets it right. If dead kits are heartbreak and tragedy to you, maybe you'd be better off not putting yourself (and her) through it!
I had a Jersey Wooly that I had to re-breed several times before she raised a litter. If she had made total hash of things twice, I'd have quit at two, but each time she got closer to getting it right, so I kept thinking "next time will be the one!" When she finally had "warm and wigglies" in her nestbox, I could have cheered!
There are many reasons for removing a doe from the gene pool. I had a Mini Rex doe that would kindle a nice litter of 6 or7, but only manage to raise 1 or 2. After this had happened a couple of times, I got really, intensively involved with the next litter, to see if I could figure out what was going on. It turned out that this doe only had 2 functional teats, the rest of her udder wasn't producing any milk at all. After all that "interference" from me, the doe abandoned her litter, and I wound up having to foster them on another doe. I bred her only a couple times after that, always at the same time as several other does, and let someone else raise her babies. Fortunately, this doe had a friendly personality. Once I had gotten "the keeper" from her, I gave her away to a pet home.