How do you know when you are done having kids?

How do you know when you are done having babies? I am so torn! I had two babies young! My son will be 5 in December, I had him at 18, my daughter will be 4 in May, I had her 17 months after my son! I love them to death. My husband wants one more ATLEAST! I am not turned off by the idea but also not 100%! I am fulfilled with how it is now, but then again, I would also like another I think! I just don’t know how to know if I am done.