On average, Chipmunks have about four to five babies in each litter, and they usually mate twice a year. This means that they can have eight to ten babies in one year. For more information about chipmunks and their reproduction, check out this site - http://www.ask.com/wiki/Chipmunk
Depending on the chipmunk, there can by anywhere from 2 to 7 babies every pregnancy. Most have an average of about 5 young per litter. However, breeding can also shorten the mother chipmunk's lifespan, so it isn't always advisable to constantly breed chipmunks in captivity.