LLMs can’t count, you’re asking the model to do something it cannot.
There are ways to make it easier for the model to do what you want it to, but there is no way to ensure it will never generate 11 or 10.
@Diet has already given you the only correct answer, ask for the number you want (or maybe more) count the number you get, then ask for more until you get the correct total (or more) and keep what you need.