Can OnCollisionEnter2D contain more than 1 collision?

It should be clear it does not because the callback only refers to a collider pair which is what a collision is in any physics engine. It does however return multiple contact points.

The order is undefined because sorting the callbacks into some arbitrary order is wasted CPU time. In the same way as you shouldn't rely the order of the Update/FixedUpdate called on GameObjects, you shouldn't for collision/trigger callbacks. Of course if you want that you are free to gather all the callbacks then sort and handle them yourself. Alternately don't rely on callbacks and just ask for contacts.

 

I'm not sure I follow what you mean by "simultaneous collisions". A collision is defined by two colliders in contact which can have multiple contact points. There's no directionality here and these are calculated sequentially so there's no one collision happening before another. For a collision pair, the callback happens on each collider to inform each. It's also called on the attached RB of each if the RB isn't on the same GO.

When a collider pair come into contact they can produce mutiple contact points. These are provided in each call and are the same for each collider because it's the contact points they share. If a collide is in contact with ten colliders then you'll get ten callbacks.

As I said though, you are free to use callbacks individually and/or gather them up or just query what a collider is touching (or if it is touching anything). Lots of options but yeah, you should not rely on an order because collisions are effectively random.