As of right now, you get a magic band for every resort stay. Split a stay between 3 resorts, and you will all get 3 magic bands. Now to top it off, they even sell special ones at the parks, Frozen, etc.
You only NEED one. If you are getting multiple, make them a little different Different colors, different printing, etc. I have so many i put my name and the year, or name and resort.
You link your AP to a user in
MDE. That user has magic band(s) associated with him or her. That is how the AP's are "put" onto a magic band. (As pointed out, they are not actually put onto the band, just associated with that band's unique id)
But yes, seriously, different colors!
EDIT:
Are you sure you get one for your APs? I just get a confirmation number, and go to MDE and link it. If you do actually get one it breaks down like this:
In your MDE account you have all your family.
Tink'sMom has magic band 1 and magic band 2.
On a disney computer somewhere:
Tink's Mom has an AP
Tink's Mom has a Fast Pass For toy story on thursday
A dinner ressie at Whatever
Tink's mom can open hotel room Wilderness Lodge, Room 7
Now go anywhere, scan magic band one, and it will know all those things. scan magic band 2, it will know all those things. It just tells the scanner that Tink's Mom is scanning her band.