The only way is to have more than one wizard then you can have more than one minion but you yourself if you summon another minion while one is still battling it will just replace that minion.
While they are not true minions you can hire crown hirelings untill you run out of crowns or all player spots are full . Other than that you can have only one minion per player at a time . Atleast as far as I know ,maybe some one know a cheat ,but it is not me.
