Flaminica
AJ said:any that you don't use would be considered a collection to me. So it could be as low as one.
This argument is incumbent on the individual relationship to the decks themselves. Are they objets d'art or are they tools? If one owns more then three drill bits or more then three sizes of saucepans it doesn't make you a collector of drill bits and saucepans. They're tools - you need more then one for different jobs.
Acquire a few decks for different jobs, say a couple of RWSs, a TdM, a Thoth and a couple of well-meant gifts and it makes you a tool user. When you start accumulating multiple copies of the same deck to compare lamination or colour separation, or if you contemplate - even for an instant - dropping five grand on eBay, then you're a collector.