In your Dashboard under Event Content > Admin Settings, we have a section where you can add Check-in Volunteer and Staff. They will simply obtain the Check-in feature in the App and no other privileges.
There is no limit to the amount of volunteers (or admins for that matter) that you can have for an event. However you should not need to many volunteers. With the ease of the check-in feature you can easily check in hundreds of attendees quickly and efficiently with only a handful of check-in staff.