Why do the 2-star items have a "1" next to them?

That's the number of the item that the vendor has in stock and available to you.

Most vendors are randomized per/player. There is no restocking, the limited quantities only affect you, though it is possible for the randomized offerings to be similar, or even identical on a day-to-day basis. Stocks from the vendors in town reset and randomize every day at midnight server time.

The exception to this is the traveling merchant, Basil, available on the "Clockwork Terminal". These offerings are the same per party, but again, quantities exist separately for each player. There's no need to "rush" over to get the best recipes - if Basil offers it to one player, he offers it to all players.

The further exception is that if a player joins the party on a clockwork terminal level or the level before, the quantity of all available recipes is set to 0. This is to prevent someone getting a potentially valuable recipe without investing any energy (since it costs 0 energy to join on a clockwork terminal level or take the elevator down to the terminal) into a dungeon.

That's the number of them the vendor has in stock. You see it on recipe vendors as well.

For most vendors, I believe they restock almost immediately... it's the in-dungeon vendors it makes a difference on.