I ask myself this question as I order two more decks: Crystal Visions Tarot and Tea Leaf Fortune Cards.
Sounds like a trick question to me. In my defense, the second deck is fortune cards.
Seriously, you can't use them all at once, but you can learn from each deck. Also, you will find special connections with certain cards. You can always gift the ones you decide you don't bond with.
My list of deck grows longer with each year, but my two main decks remain Legacy of the Divine and Mystic Dreamer Tarot. With these two decks, I found an automatic connection. I do however enjoy all of my decks, and have favorite cards in each.
What decks do you own? Do you have a favorite? Do you use them all?