There is only one queen of each color is a chess set. When a pawn gets promoted, say to a queen, where does that queen come from?
Most over the board sets come with 4 queens total (2 of each color) for exactly this situation. It's quite likely that at some point in your games, you'll need to have 2 queens on the board simultaneously, so they give you the main queen and a spare.
If you really need more than 4 queens, you can always "steal" another piece from your neighbors' game so that you have enough pieces to continue playing. Just make sure you give it back when one gets captured or they'll be mad with you!
Of course, it's possible to have 9 queens simultaneously, but it would be unnecessary for any chess set to give you this much redundancy on the off chance you decide to troll your opponents and promote every piece to a queen. Same thing with underpromoting so that you have 10 knights, 10 bishops, or 10 rooks...