For every purchase any master or master like power that generates a minion will have their limit increased by 1.
For powers such as (Master) Specter this will allow you to create 1 more projection every day, for powers such as (Master) Friendbringers this will allow you to create one more friendbringer. For powers such as (Master) Hivemind it increases how many you summon a day by the same amount. For powers like (Master) Undead you gain one more slot.