15 on a single flight sounds fairly high risk to me. Even if someone in customer services says it's OK if there's a load of stuff to go in the hold, and they need to make some room then the bikes are going to be a strong candidate to get bumped. I suspect the rule is 'we'll carry as many bikes as we have room for - and if we haven't got room we'll put them on the next available flight'.
Maybe book split the group between (say) three flights?
It might even be worth thinking about hiring a van and driving them there - yes I know a pain, but it would mean you'd have a support vehicle for the trip. Or there might even be a transport/removal firm that would do it.