How many stones did Israel take from the middle of the Jordan as a memorial?

Answer: Twelve

They took up twelve stones from the middle of the Jordan, one for each tribe of Israel, to serve as a memorial.

“and command them: ‘Take up for yourselves twelve stones from the middle of the Jordan where the priests were standing, carry them with you, and set them down in the place where you spend the night.’””— Joshua 4:3 (BSB)