Nope. You're all good.
In an SG500 stack, there are three roles: master, backup and slave. The master currently holds the management role of the stack. The backup is the switch that will take over the management role if the master fails. Slaves are remaining stack members. As long as the master is active, the backup will essentially function as a slave.
If you only have two switches, there will be a master and a backup with no slaves at all.