Their are 20 recruited walk-ons allowed at the FBS level (105 players total allowed for fall camp, per NCAA rules), and then there are usually a handful of traditional or non-recruited walk-ons that don't start practice until the first day of school.
As to the rest, the 5-7 number means high level D1 players that are good enough to play for anyone. Iowa and ISU will always have to fill out the remainder of their rosters with "reach" players or walk-on types. You hope that these guys can develop and fill in the gaps for your recruiting misses on scholarship players. When they do, you can have a solid team. When they don't, you go 4-8. Take a look at the all-time win-loss records for both Iowa and ISU to see how that typically works out.