Re:how many cards should i have?
Only to use the first deck and replace the cards with the second deck as they wear out.
You can't really play with two decks at once. You need to have one suit of 9 cards per player. Doubling the number of cards in a suit will either increase hand size and make it more difficult to hold and keep track of, or, if less suits are used, make it far too quick to complete a set.
I suppose you could try a variant in which the lower-valued suits have one or two extra cards, making it easier to get a set of 9. That might be fun, as those would be a bit more tempting to try, as they'd be a bit easier. However, I think the whole guess and second-guess of what suit you can complete the quickest works well enough in the original. If you do try something, let us know how it went.