I spent the first 7 years of my tarot life with no more than 2 decks at a time - a single one for the first 3-4 months (RWS) to which I added the Morgan-Greer later. Altogether, during those 7 years, I owned 5 decks, but as I said - never more than two together (I gave away the others). Then came the 7 year-itch and I started to want to spread my tarot wings a little. I bought 3-4 more. From about the middle of last year, I have been collecting more seriously, though still haphazardly (that is, as the mood takes me, and without a system). I must have close to 50 by now - tarot and oracles. I don't read with all my decks, but I do look at them all and spend time with them. Those I don't really get on with for anything I trade away. Some decks I just like to sit with, some I like to study, and some I read with occasionally, to exercise the intuitive juices. Maybe after the second round of 7 years I'll get rid of them all and come back to my simpler ways...or maybe not.
I would say, though, that unless you are a very focussed person naturally, it's best to start with one deck and stick with 1-2 for a few years before you branch out with more decks. Too many would-be tarot readers don't take the time to deepen their knowledge, and just skate along the surface of dozens of decks. Unless, of course, you like tarot primarily for collecting and for the multiplicity of the art work (wink to gregory).