Each Minion has the same health amount as the Boss.
Rulebook p23 wrote:
A Boss and its Minions share the same characteristics and Enchantments, as shown on their Enemy card. The Health of each of them is managed on an individual basis.
Rulebook p36 wrote:
Each figure in a Mob has the same Health, as indicated on the Enemy card.See also examples p35 and p36 with Dwarf Defenders/Warriors with Health 2.