Is this oven good for Neapolitan pizza?

No. Its 550°F maximum is well below the 700–900°F a Neapolitan oven needs. It suits New York-style, pan and thicker pies baked at moderate heat.

How many pizzas can it hold at once?

It has two decks, each about 24 by 24 inches, so you can bake on both levels at once. Exact pie count depends on size and how much room you leave between them.

Does it run on natural gas or propane?

It ships for natural gas and is field-convertible, so a technician can switch it to propane. It needs a 3/4-inch gas connection.

What are the differences between the three sizes?

The options differ in width: 43 5/16, 55 13/16 and 55 1/8 inches, with model numbers MS45G-2, MS56DG-2 and MS56RG-2. Height, depth and deck size are the same across them.

Can it sit on a standard counter?

It is a countertop unit, but it weighs about 595 lb, so the counter and support must be rated for that load. Check clearance for heat and ventilation.