You can put up to 4 cards with "Life Recovery" and "Void Bringer" each in a deck.
No, it is not necessary to put cards with these effect in a deck by all means. However, it is recommended to put 4 of them each if you are not used to the game.
Yes, it is necessary to put that fixed amount. Any amount greater or lesser than that are not allowed.
It is not necessary to put cards with "Start Card" in a deck. However, since there is no merit for not using it, it is recommended to put at least 1 in a deck.