How many RC-522 RFID reader/writers can I use with one Arduino?

How many RC-522 RFID reader/writers can I use on a single Arduino (e.g. the Uno)? Like, is the only limit the amount of physical inputs on any Arduino or does the system become unstable with too many?

I have seen 4 at the most being used at once in examples such as this project:
Hackaday RFID project

Also, should I use a bi-directional convertor for each connected RFID reader/writer or will they function fine without them?

Thanks in advance for any advice :slight_smile:

How many RC-522 RFID reader/writers can I use on a single Arduino (e.g. the Uno)?

The maximum SPI bus length is about 40-50cm. How many of these readers do want to have in that limited range? It doesn't make the slightest sense to have more than one or two of theses readers connected.