Deck card number?

They're 30 card decks (minimum and maximum), and two maximum of any one card. There are over 300 cards to collect, meaning over 600 to build out a 'complete' collection if you want two of each.

Edit: Actually, you can only have one of each Legendary so slightly less than 600 total to collect, but if you include Golden cards then you're up near 1200 I suppose. Anyway... a lot of cards. ;D

What if you have two similar cards you wants to use in two separate decks ?
Let's say i have Arcane Missile x2 and I'm building two mage decks.
Can i use Arcane Missile x2 in those two decks

Yes, the decks you build are essentially choices of your library. Building a deck doesn't "use" that card up, it's about the strategy of the deck, not having tons of duplicates. We actually have a fast disenchant feature that'll find any duplicates above two and allow you to disenchant them all with a single click.

What if you have two similar cards you wants to use in two separate decks ?
Let's say i have Arcane Missile x2 and I'm building two mage decks.
Can i use Arcane Missile x2 in those two decks

Yes, the decks you build are essentially choices of your library. Building a deck doesn't "use" that card up, it's about the strategy of the deck, not having tons of duplicates. We actually have a fast disenchant feature that'll find any duplicates above two and allow you to disenchant them all with a single click.