The value of X is determined only as Night of the Sweets' Revenge's last ability resolves. It won't change later in the turn if the number of Foods you control changes.

Night of the Sweets' Revenge's last ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won't be affected.

Whatever you do, don't eat the delicious cards.

Food is an artifact type. Even though it appears on some creatures, it's never a creature type.

You can't sacrifice a Food to pay multiple costs. For example, you can't sacrifice a Food token to activate its own ability and also to activate Maraleaf Rider's ability.

If an effect refers to a Food, it means any Food artifact, not just a Food artifact token. For example, you can sacrifice Tough Cookie (an Artifact Creature — Food Golem) to activate Maraleaf Rider's ability (an ability with "Sacrifice a Food" in its cost).