How many Teeth?

Rachel
How quick do teeth come in? 
From the looks of baby's gums, he's getting 2 right now. I've been suspicious of teething for a while, but I can see the white bumps now. 
Our cousins baby boy got 7 within a few weeks. How common is it for baby to go from 0-several? Is it usually one at a time or is it a pretty quick thing?