Text (ON+errata): Whenever a creature enters the battlefield, you may add {1} to your mana pool for each creature you control that shares a creature type with it. [Oracle 2009/07/08]
You do get to count the creature itself, you even get one mana if it is the only creature you have in play (assuming it has at least one creature type). [Barclay 2003/02/21]
You either get zero mana or all the mana. You cannot choose an amount in between. [Onslaught FAQ 2002/09/24]
It counts up creatures of that type when the triggered ability resolves. The number of creatures of that type may change before the ability resolves. [D'Angelo 2009/07/19]
