2 or 3 kids...? How did you decide?

My husband and I are constantly on the fence about having a third child. We are financially stable, so that’s not an issue. But I worry about physically being able to handle a third pregnancy. My second pregnancy my back continually went out and with a c-section on top of that. The recovery was rough! People tell you know when you are done. But I don’t have those feelings yet...I just don’t want to be that little old lady in a nursing home wishing I had more kids. Aka regret! My big fear as well is being out numbered 2 kids to 2 parents! How did you know when you were done? Or wanted more children?