You have 15 costumes and only 3-4 days to show them off, right? I too, share that problem. But, you have to remember to keep yourself hydrated and taken care of before you try to bring all 15 of them out. I'm here to help you decide how you should proceed in this course of action.
Determining how many days you are going. If the con is 4 days, but you're only going 2 or it's 3 days and you're only going 1, you shouldn't have too many costume changes. You wanna spend as much time with your friends and getting pictures as possible. You don't want to spend a grand total of 3 hours on the floor, but you've had 3 changes. It means you've wasted your money for the badge, and potentially any other activities you wanted to do.
Try to limit your costume changes. Group cosplays are becoming super popular and more widely practiced for certain fandoms, but you also need to remember to take care of yourself. If you are in three groups, and they all want to meet within an hour of each other on Saturday, I would honestly say pass. I was supposed to be part of a Sailor Moon Baddies group at ASTL this upcoming month, and with my competition build, I would have had to go get ready, get judged, wipe off the makeup, do the photoshoot with the Baddies, get back into the competition build, and then do the competition. It wasn't feasible emotionally, let alone physically for me. So I politely declined. That doesn't mean I won't eventually do Baddies with them, it just won't be at this specific convention.
The way I try to justify costume changes is spending at least 3 hours in the specific costume and how difficult it would be to change makeup, outfit, and comfort. I have my fair share of uncomfortable costumes, but if I can help it, it'll be cozy. If I do happen to have a costume change, it most likely will be Saturday, since I'll have the longest day to spend the most amount of time in my cosplays. That doesn't mean I haven't had a costume change on Friday or Sunday, I just prefer to keep it to Saturday.
Also keep in mind if this is your only con for the year. If you have a couple costume changes because you aren't going to anymore cons, that's understandable. But, I have seen people posting lineups of 8 costumes for 3 days. Which I feel (personally, it is your life) is excessive and just ruins the con for you. If you have a huge bulky cosplay that is uncomfortable to wear, absolutely have a backup just in case. You deserve to be comfortable after hauling that around for a few hours.
Lay out your cosplays before the con, make sure you have all your pieces and if you don't, make a list of the things you have to make/buy to replace them. The lists can get long, so try to choose cosplays you won't have to replace much on, unless you can space it out and have time to remake the pieces. Cosplay is an expensive hobby, so make sure you aren't skipping out on your bills just to finish pieces.
Determine if there are any meetups you want to go to/be a part of. Help that decide what cosplays you want to bring. For example, Friday night of ASTL, my STL Hero Academia group is holding a meetup for the My Hero shoot. So I decided that I was only going to wear Hero Bakugo for the shoot. I may bring his gym uniform if I'm exhausted after a while, but who knows? If you have a meetup you'd like to be at, help that make your decision. But don't overdo it by trying to be part of 3 meetups that are one right after the other. That's super stressful and your mental health is more important than being part of some photos.
Don't stress yourself out trying to make a ton of cosplays to prepare. I see my friends doing this and it hurts to see them bring themselves down to meet these ungodly expectations they put on themselves. You are human and it is okay to breathe. It's also okay to only have one cosplay for the whole weekend. Just remember to bring Fabreeze to sanitize your cosplay, especially if you are a sweatier person. You don't want to be one of those guys. Actually, even if you have multiple cosplays, bring fabreeze so you can spray down your cosplays after your done, in case you can't wash them, or they'll be waiting in the hotel closet for the next couple days until you can get home to wash them properly.
If I do happen to see you at Anime St. Louis in the next couple weeks, definitely come say hi! I'll be in Hero Bakugou Friday, Saturday I will be with my S.O. in Maya and Zer0 from Borderlands 2 for the cosplay competition, and Sunday, I will be in Rukia and Ichigo from Bleach's first anime opening. I hope you have a great time and an easier time picking a choosing your cosplays for your next Con Lineup!