What exactly do you mean by "multiple" tag reading ? Do you want to detect continously the precense (and non-precense) of multiple tags ? Is multiple 2 or 10 or more ?
You will rarely find information about multiple tag handling with NFC readers because that is not a normal usecase.
I'll quote someone from the internet because I agree with what they say.
David Maier, Professor of Computer Science
Answered Mar 15, 2015
Hard to tell -- there are several factors at play.
In the single-tag case, the antennas of the reader and the passive tag are essentially operating like the two windings of a transformer, with electromagnetic induction coupling them. The system is tuned to a particular frequency (13.56 Mhz). If there are two tags near each other, particularly in a stack, they can experience something called "mutual detuning", which distorts their frequency response. So you might not get a reading from either tag.
A second issue is that the tags draw their power from the reader's RF signal. If there are a lot of tags in the field of the reader, there might be insufficient power to trigger any one of them.
If you do get a response from several of them, there could still be a problem if the reader software is only set up to deal with one tag at a time. In theory, the reader can connect with the first tag it detects, then issue a "halt" command to that tag, and try to read another. However, apparently some SDKs for reader devices assume one tag at a time and don't support this mode of operation. Others can handle tag collisions.
There is also this thread with more information about how he Adafruit Shield is designed. ( Related to an older version using PN532 chip and not PN512 which I think is in the newer model but I belive they behave mostly the same. )
https://forums.adafruit.com/viewtopic.php?f=31&t=35050
Is there an option to use other RFID standards than ISO14443 (aka NFC) ? They might be better suited to your specific usecase.
