By "more mercenary space" I think you mean archive space. You can buy archive expanders from the shop or get them from other players. You can also get them from events like leveling up in which you will get them from things like leveling up to certain levels (10, 20, 30, etc.) in the form of a gift.
If you mean in your party then you can also get an item to unlock the last party from the cash shop. Sorry I don't know the name of the item itself.