During the 2015-2016 school year, a total of 6,986 students were tested in Iowa. Of the 5,191 juniors and seniors that were tested, only 30.90% were tested under Option 8. The remaining juniors and seniors had their test results sent to recruiters without parental consent.
The 20 largest testing sessions assessed a total of 1,744 students. 220 sessions tested 10 students or fewer.
The ASVAB test was conducted in 203 schools across the state. 24 of these schools report that they made the test mandatory. Consequently, 764 students are confirmed to have been required to take the ASVAB; there may be more, judging from the numbers in some schools. See Iowa's database here.