Sex - how often?

My husband and I have a weekly quota we like to hit: have sex 4 times. I think this is enough, but I'm curious if others would agree. Is our goal high or low? How often do you have sex each week! Do you and your husbands have a weekly goal? 
Please note, some weeks we don't hit the goal and other weeks we exceed it. It's a loose goal - want to keep things fun not forced. Just want to make sure we're maintaining a healthy sex life. 
Sorry if that's too personal of a question!