By default, a backup master is treated just like another regionserver. So it can host as many regions as other regionserver does. When the backup master becomes the active one, region balancer needs to move those user regions on this master to other region servers. To minimize the impact, it's better not to assign too many regions on backup masters. It may not be good to leave the backup masters idle and not host any region either.
We should make this adjustable so that users can control how many regions to assign to each backup master.