Because Wastes is a basic land, you can include as many of them as you like in your Constructed decks.

In Limited events (including Sealed Deck and Booster Draft), Wastes must be in your card pool to be included in your deck. You can't add Wastes to your card pool in the same way that you can add other basic lands.

Wastes is not a land type. If something asks you to name a land type, you can't choose Wastes.