White has a queen on c7 and another on b5 that was a pawn a few moves ago, and the second one is every bit as legal as the first.
g That move has to be written Qcb7 rather than Qb7, because both queens can reach b7 and the notation has to say which one went. If that looks odd, disambiguating moves explains the convention in a minute.
Nine, if every pawn gets through
The Laws are explicit that your choice of promotion piece is not restricted to pieces that have already been captured. Nothing anywhere caps the number of queens on the board. So one original plus eight promoted pawns is nine queens for one side, and eighteen queens between the two players, which is a legal position and a completely absurd one.
Real games are more modest. Two queens turns up in endgames often, usually in a race where both sides queen and one gets there first. Three appears occasionally. I have never seen four over the board and I have never met anyone who has.
Two queens are a mating machine and a stalemate trap
The good news is that two queens mate a lone king almost instantly. They cover ranks, files and diagonals in overlapping sheets, and you can drive the king to the edge with one while the other cuts him off, without your own king doing anything.
The bad news is the same fact from the other side. Two queens take away so many squares that a lone king often ends up with nowhere to go while not being in check, which is stalemate and an instant half point for the player who was losing. The habit that fixes it is small: before any quiet queen move, count the enemy king’s squares, and if the count is zero, play something else or give check. How to avoid stalemate has the drill.
The wooden problem
Chess sets come with one queen a side. When your pawn reaches a8 in a tournament game, the correct move is to stop the clock and ask the arbiter for a spare, not to improvise, and above all not to turn a rook upside down and hope: under FIDE practice an upside-down rook is a rook, and you may find yourself playing an ending with five points where you thought you had nine. The details are on promoting without a spare queen.
Online there is no problem at all. The board draws as many queens as you make, and the only thing to watch is that your auto-queen setting does not promote for you in a position where a knight ends the game faster. That case is covered under when a queen is the wrong choice, and it is worth knowing before it costs you a point in a promotion puzzle or, worse, a real game.