There are total of six henchmen available in the game.
These henchmen will aid you in battle but at a price.
Cost - 100 Class - warrior Level - 5 Location - Red lion inn,Taryn ridge Wield - balanced longsword
2)Schmendrikk the great
Cost - 500 Class - Mage Level - 16 Location - Slug and lettuce inn, Cold sands hold Wield - studded staff Spells used - Dazzle,Fire blast
Cost - 1000 Class - warrior Level - 23 Location - Redrock Bay Wield - axe
All 3 of them have their level fixed and it doesn't go further. Sadly you have to face merificus alone.
4)Mages can summon fire elementals who level with skill progression and can be dismissed with ease.
5)Rangers can summon spirit wolves who are near to the same level as them but they can't be dismissed unless killed as they only bite and howl.
6)Last but not the least, remember Feskers the lost dog ? Till the time you don't speak to Aven the little boy he is a henchman and can be a great help to battle wolves at the beginning of the game.
As a Mage or a Ranger you can hire two Mercs and summon your aide and let them brawl the foe and have a puff of smoke and shoot the enemy at ease.
As a warrior or paladin there is little point of having henchmen as your hacks,slashes & bashes do the job.
As a rogue since you need to be stealthed for maximum damage there seems to be no point of having a henchman.
Henchmen won't attack you even if you use an offensive skill on them but the warning will flash repeatedly.
Don't take them near ledges and traps as they don't mind their step.