How many words is average?

I'm trying to find some good sources of how many words/phrases are average at each age. I feel like I have no real concept of what is "average" because the kids I'm around tend to be either very verbal or very not verbal. For example my cousin Joe at the age of 3, could explain that he didn't want any of the pages of his dinasaur book read that contained information about carnivores (yes, he actually used that word) because they scared him. You can't tell me that the average 3 year old knows that (or maybe even the average 7 year old). On the other end of the spectrum one of my nephews was barely talking at 3. (He's now 7, and doing much better verbally, but I would have to guess still behind.)
So far the words/phrases Carter has are:
Mama
Dada
Umpa (His word for Grandpa)
Is (His word for Chris)
Baba- bottle
What is it? - (I didn't believe this one at first, but he says it all the time when pointing to things and it's always in the correct context)
It's done- (I'm not completely sure about this one because we've only heard it a couple times)
Bye-bye
I'm sure there are a couple more, but these are the only ones I can think of right now.
I have a feeling that we will be doing much of his learning using music. He loves music!