I'm in the same boat! I know we are having a destination wedding, and others are aware, but at this point we don't know who is coming since we don't even have a date/hotel picked out. I have a group of 3 girlfriends and we are all close, but realistically, I doubt all three of them will be coming. I would love it if they were, but I am fully prepared for the fact that they may not be able to.
What my FI and I have done, is we have one couple that are without a doubt coming no matter where, when, or how much the wedding is. It's his childhood best friend, and his wife, who I've become very close with over the past couple of years - so we are going to have the two of them stand up for us. This way we have at least two people standing up with us, and I am just going to be flexible (or try to be!) with everyone else.
I am going to ask my three girls, but make it clear to them that I understand that they may not be able to come, and there are no hard feelings at ALL. I plan on being flexible about BM dresses (pick a colour, they pick the dress), so even if they decided at the last minute to come, they could join the wedding party. I suppose I also have to be flexible as far as not being concerned about the numbers matching up on both sides....there may be more on my fiances side, than on mine, or vice-versa.
This is something I've struggled with, and I hope it ends up working out ok!