The bonus is only +1/+1 per applicable creature, even if it shares more than one creature type with the equipped creature. For example, if the equipped creature is a Human Shaman and you control a Human Warrior, a Goblin Shaman, and another Human Shaman, the equipped creature will get +3/+3.

The bonus will change as the creatures you control change. This may cause the equipped creature to lose toughness and die because of damage dealt earlier in the turn.