Which games have the option to buy multiple copies in one purchase?

None, you can buy 2-packs, 3-pack or 4-packs if it's offered on the store page (and in most cases, packs can only be bought for the library, meaning one copy will be activated immediately on the account that buys the game while the other copies will be sored in the buyer's inventory).

Other than that you don't have the option to put multiple game copies of the same game into your basket.

Usually it is games that have a co-op feature that are offered in multi-packs.