I am not completely sure how it all works, but here is my first shot at it. You get one free functioning Collector license with each seat of ArcGIS. I have ArcGIS Map and I get one license for one device (I think) and 100 credits. You use up credits by loading things up to ARCGIS Online - more details here:
ArcGIS Online | Service Credits
Additional credits cost $100 for 1,000 credits. I am just trying to lean how to use this extremely complicated program and I have used up 45 of my 100 credits.
TIP - only upload feature classes (layers) and not imagery to ArcGIS Online. You can side load basemaps to the device just by sending them to your device as .tpk files - Warning - your imagery must have projection information or the imagery will not show up. You can add the projection using the Tool Box.
You can also buy additional licenses above the one free license per seat, but it is very expensive, about $500/user if you need 5 users and less if you have many more users. More details here:
ArcGIS Online | Pricing