This ability counts creatures, not creature types. For example, if you attack with five creatures -- an Elf Shaman, an Elf Warrior, a Goblin Shaman, an Elemental, and a creature with all creature types -- the ability will trigger five times. Those creatures will get +3/+0, +2/+0, +2/+0, +1/+0, and +4/+0, respectively.

In a Two-Headed Giant game, only creatures you control trigger the ability and get the bonus, but your teammate's attacking creatures are included in the calculation of those bonuses.