There's no restriction on how many Class permanents you can control, whether they're the same or different classes. Each Class permanent tracks its own level separately.

You can't activate an ability that advances a Class to a particular level if that Class is already that level or higher. For example, if a Class is already level 2, you can't activate the ability that advances it to level 2 again.

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

An ability that advances a Class to a higher level is a normal activated ability. It uses the stack and can be responded to.

Some spells and abilities that create Food tokens may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. You won't create any Food tokens.

Class cards have multiple text box sections. Other than the top section, each section contains a class level bar that includes a cost and a level number. Each represents an activated ability with the listed cost that advances the Class to the level listed in that bar.

Gaining a level won't remove abilities that a Class had at a previous level.

The value of X is calculated each time Party Dude's last ability resolves. Changing the number of cards in your hand after the ability resolves won't change the bonus granted to that creature that turn.

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

Each Class starts at level 1 and has the abilities in the top section of its text box. Once a Class advances to level 2 or level 3, it has all of the abilities listed in the associated section of its text box as well.

Whatever you do, don't eat the cards. (Also, don't make the pizzas pictured on the cards. You don't have the digestive system of a Ninja Turtle.)