How many expected unique cards in 52 of 6 decks?

6 decks of 52 cards are shuffled together randomly. How many UNIQUE
cards (i.e. rank and suit) would you then expect in the first 52 dealt?

9 is the minimum, and 52 is the maximum. What is the expected number?

So how do i figure this out for 6 decks or for N decks? What about an
infinite deck?