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.

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.