User blog comment:B214/High School DxD EX/@comment-24583889-20160310182046/@comment-87.9.180.51-20160508005334

probably more than 20 since 20 kids were the ones old enough to travel back in time, so is probable that Issei has other childrens who were too little to travel back in time and/or fight real battles (probably between 0 and 10/12 years)