Is it normal that we don't have sex very much ?

Me and my wife have been married 4 years. We have 2 kids. We don't have sex very much. We've only did it maybe 4 or 5 times since we been married. We did it after the first couple months we were dating. We did it after our daughter was born she's gonna be 4 in December. We do it once in a while when we want to. We did it a few months ago now we're having another baby in February. She's been wanting to do it the last few nights. If she wants it we do it. We don't do it as much as we use to. Is it normal we don't do it as much ? How often should we do it ?