You may choose not to sacrifice any artifacts for the devour artifact ability.

If you cast this as a spell, you choose how many and which artifacts to devour as part of the resolution of the spell. (It can't be countered at this point.) The same is true of a spell or ability that lets you put a creature with devour artifact onto the battlefield.

Devour artifact is a variant of the devour ability. It allows you to sacrifice artifacts rather than creatures, but otherwise functions identically to devour.

All artifacts devoured this way are sacrificed at the same time.

You may sacrifice only artifacts that are already on the battlefield. If multiple creatures with devour abilities are entering the battlefield under your control at the same time, they can't devour the same objects. They can't devour each other, themselves, or any other objects entering the battlefield at the same time.