When we were just there we had a split stay and were warned when we checked in at the second resort that they have been having issues with having guests have "too many".
We had five each, one from each of our resorts that trip (no option to decline when we customized them), one AP, and one that we had paid for from a previous trip when no one had one. I thought that was not "too many" and left them all active.
We did not have any issues with using any magic bands at any time to enter the parks or ride rides.
BUT: At our first resort, all our magic bands worked on the door, including the ones we had prior to this trip. At our second and third resorts, magic bands did not open our room doors unless they had been specifically re-coded at the desk. That is, all bands worked at the park, not all bands worked on the room door.
Based on some other mutterings from the various CMs we encountered during this process and a few things I know from my real life, I subsequently formed the suspicion that "too many bands" means "we don't want to recode each band individually at the resort and we don't want to hear you complain about or be frustrated by them not working". I also have a suspicion (not tested but can be tested by anyone who cares to and has a extra magic band that worked on an old room but doesn't work on a new room), that deactivating the band releases any room coding that is associated with it. So a shortcut to getting all your bands to work with the room might be to deactivate them and reactivate them in
MDE after you check in at your new resort (might have to wait a day, might not for the old resort hold to vanish from the system), which would allow you to have all your bands active without having the overhead of waiting at the front desk while some CM recodes 30 bands.