If they are all yours in your MDE account, they are essentially all identical (electronically), essentially clones of each other. This means that although you have 15, they are all the same to the system so you could use any of them interchangeably.I have 15 MB's on my MDE and 8 are active. Five of them I have blinged and I want to bling a sixth one with a Christmas theme. Is that too many to have at once?
A case of trying to solve a problem by choosing to “fix” the wrong issue.
But, if the issue is of concern, just inactivate the oldest MB(s) that you no longer intend to use.
See Robo's comment above.My DH and I each had 18 MagicBands and multiple cards from AP's and parties. We've had some glitches with all those bands, even though I never had more than about 5 or 6 active at once. The biggest mess was trying to get into our room at the BC in July. The CM had to spend a lot of time loading the bands that we had brought. I don't know that the problem was but I later read that the resort door locks load all the MB numbers into their memory, and that can cause problems. I don't know if that is right or not, but I decided to call the IT dept and have them delete all of the old MBs and cards that we never intend to use again. Now we are down to about 5 or less MB's and we didn't have a single glitch during our trip last month.
I read Robo's comments.See Robo's comment above.
Their systems have issues. MB aren't the cause of them. Dealing with them won't be the fix of the issues. They may be a bandaid for the moment but the issues remain.
But our experience has been a little different, for whatever reason. 
My DH and I each had 18 MagicBands and multiple cards from AP's and parties. We've had some glitches with all those bands, even though I never had more than about 5 or 6 active at once. The biggest mess was trying to get into our room at the BC in July. The CM had to spend a lot of time loading the bands that we had brought. I don't know that the problem was but I later read that the resort door locks load all the MB numbers into their memory, and that can cause problems. I don't know if that is right or not, but I decided to call the IT dept and have them delete all of the old MBs and cards that we never intend to use again. Now we are down to about 5 or less MB's and we didn't have a single glitch during our trip last month.