How many bees are in a hive?

Under normal circumstances there will only ever be one queen in a honey bee colony. The bees will not tolerate a second queen and if you tried to introduce one they would kill her.
There is one exception. If a queen is nearing the end of her laying life, queen cells will be made by the workers, and a new queen hatched off. For a short period it is possible for both the mother and daughter queens to be seen together. A few days later the old queen would be gone. This process is called supercedure.