If you control more than one Ragemonger, the cost reduction is cumulative.

You apply cost reduction effects after other cost modifiers, so Ragemonger can reduce additional costs or alternative costs of Minotaur spells if they include {B} and/or {R}.

If a spell has hybrid mana symbols in its mana cost, you choose which half you will be paying before determining the total cost. If you choose to pay such a cost with {B} or {R}, Ragemonger can reduce that part of the cost.