I would definitly not ask her to be a backup. I think you have to work on convencing your FI that it's ok to have mismatched numbers. Tell him people do it all the time now. I think guys tend to have strong opinions about wedding stuff when they haven't really thought about it. It's more important to not hurt feelings than to have your wedding look a certain way. Also, if you piss off a bridesmaid, they can really make things difficult for you. I've seen disgruntled bridesmaid not bother getting their dress on time or not taking care of alterations. I've seen them up infront of everyone in a mess of a dress. it will be all wrinkled, too big, or too long. Just because they got angry witht he bride & didn't want to deal with any more bridesmaid stuff. Also as someone who has been a bridesmaid 9 times, I know it's really easy to get your feelings hurt. it's a tricky situation and if a bride doens't handle it right you can really get hurt. It can easily go from an honor to feeling bullied. They can pick out dresses that are too expensive, demand too much of your time, or boss you around.