Over 100 characters in The Simpsons are voiced by 12 very talented people — and here's a breakdown of who does what. Click on the top right magnifying glass to enlarge.
The image, which comes from @Jishai, might not be a surprise for hardcore Simpsons fans, but it's still fun to see everything laid out like this.
Compare to the cast of South Park, which is primarily voiced by four people:
Dan Castellaneta and Trey Parker, doing work!