When can I stop breeding her?

I have a beautiful, loving, 6 year old pully/bull breed mix who is currently pregnant with our fifth litter. We absolutely love the babies and are sad to see them go when they are done nursing. The problem is that we are not a puppy mill and can only raise two litters a year.<br><br>She’s still very young to retire and I don’t want to spay her. Are there other options?