How can you tell when your ginny pig is going to have a baby? and how many can they have?

Gestation for a GUINEA pig is any where from 60-70 days with an average of 63 days. They can have any where from 2 on up. I've even heard of one having 6. Out of the 3 litters that I have had over the past year the biggest litter was 4 and the smallest was 2