Checkmate - Lycée Bellevue (Alès) Colegiul National Mihail Eminescu (Satu Mare - Roumanie)

We consider a 8x8 chessboard. We place a queen on it. Then, we place a second queen without her being eaten by the first one, etc.
How many queens can we put on the chessboard? Explain.
Same question with the knight, the bishop and another piece for which we can
choose its move.