Anti-collision Capabilities of MFRC522
The anticollision for Mifare Type technology is capable of selecting as much transponders as the UID length is. For a 7 Byte Type UID (E.g. Desfire) there could be 256^7 tags selected. Here You find the reference for this: https://www.nxp.com/docs/en/application-note/AN10833.pdf
Because of power Limitation, practically you can not read out more than 5-10 transponders.