You buy henchmen levels 20-100 in the crownshop all schools. The higher the level the higher the cost of crowns. If you want to buy one, you'll have to be in a fight first. Click crownshop and you can buy one there and it will summon automatically. They only last for one fight though, so when you do buy one, make sure you really need it!
Well you can buy henchmen from the crowns shop. They're basically just like really powerful minions. You can buy henchmen from level 20 all the way up to level 100. Although the henchmen can't actually cast level 100 spells if they're level 100, it just means they're more like a level 60 or 70 wizard.
Unlike minions, you can summon as many as you want until your side of the team is full. So let's say on your team there is you and one other person. You could buy two henchmen. Also, do keep in mind: henchmen can die, and if they do, they can't be brought back to life. They also can't be used in PvP. Hope I helped!