I am married with 5 children, my ex husband wants to claim 2 of my kids, does it benefit me to claim 5 kidsand file jointly with my now husband or should I just let my ex claim the other 2 kids?

It almost always benefits you to file jointly with your current husband and claim as many of your children as possible. If all 5 children live with you (not with your ex) then you are entitled to claim them, unless your divorce decree states that your ex can claim them.