Since these are your first decoys, simply go with mallards as they tend to be less expensive and will attract most species of puddle ducks. I recommend water keel so that when you decide to carry them, they will be MUCH lighter. The downside to water keel is that if you are tossing them from a boat, some may not self-right once they hit the water. As for how many, a couple dozen will be enough to start with. There are, however, a lot of variables in this answer:
1) how do you transport them (in a bag on your back or in a boat). If you're carrying them 4 dozen is a lot!
2) where are you hunting (big open water such as a lake, water with standing vegetation such as a marsh, or water with lots of sand bars such as a river). big water tends to hold larger rafts of ducks, thus more decoys is probably better.
3) how are the ducks sitting naturally in that body of water? As some guys above said, a few decoys on dry ground may look more natural, but those decoys are pricey. If you aren't hunting a spot with dry ground, you don't need those decoys.
The spots I've mainly hunted this year are walk-in marshy areas with no dry ground where the ducks wanted to be. I carry in about 30 decoys with mostly mallards and a few pintail drakes to add some white for visibility. They are a mix of brands, which doesn't really matter to me.
I've killed ducks over a buddy's decoy spread where about 1/3 of the 100 or so decoys are just painted 2-liter bottles several times, so I can't say that highly realistic decoys are a necessity :)
"not all those who wander are lost"
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."