The Vic-20, like many home computers of its time, had a cartridge slot that could be used both as a general expansion slot, and for game cartridges like a console. Game cartridges on the Commodore machines faded away later, but seemed to be quite popular at that time: http://www.zimmers.net/anonftp/pub/cbm/vic20/Cartlist-1.9m.txt
Console manufacturers plan on eventually selling several cartridges per machine; this is both natural from the customer viewpoint, and central to the business model from the manufacturer viewpoint, though to Commodore they were more of a sideline.
How many game cartridges were sold per Vic, or in total across all ~2.5 million Vics? I would be interested in either a number by Commodore alone, or by Commodore plus third parties.