Is it possible to make more than 1 giveaway of the same game ?

yes

add more keys to the place were you deposit your keys when you create your Giveaaway.
or when you select gifts you can enter the number ;)

Yes. See my profile :D

Of course it is, you can either make separate giveaways, or a single one with multiple keys.
The second method would be preferred, to avoid trouble with users winning more than once the same game.
Mind that if you give away several copies of a game (either in a single giveaway or multiple ones), you would get reduced CV after the fifth copy.