We also breed once per year and feel it's better on the doe and keeps her healthier and happier.
I know that some does will have litters up to 2 times per year, that is usually pretty hard on them though and will wear out their body faster. I know of a doe who had quintuplets two times in one year! She got a nice long break after that though.
But anyway, yeah 1 time per year is usually ideal.