The PN532 can read up to 2 cards at a time - check the PN532 documentation for any more details about anti-collision support - we've dont remember seeing anything about it & anything the PN532 supports, the shield supports (altho there may not be example code for it)
From skimming the manual it looks like some of the commands will return a specific error code when there is a collision. One then has the option to send a select command to pick the desired target and then re-run the command.
Section 7.3.5 shows the InListPassiveTarget command, which lists the available targets. You are correct that the 532 can only detect up to two targets.