To add volunteers to take care of check-in only you can easily add them on your dashboard to the admin settings page under 'Volunteers'. Volunteers in this way will only have access the 'check-in' feature on the app. This does not give them access to your organizer dashboard (desktop version).
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.