Jamieson, Fausset and Brown on Judges 10:4

4. he had thirty sons that rode on thirty ass colts--This is a characteristic trait of Eastern manners in those early times; and the grant of a village to each of his thirty sons was a striking proof of his extensive possessions. His having thirty sons is no conclusive evidence that he had more than one wife, much less that he had more than one at a time. There are instances, in this country, of men having as many children by two successive wives.