How Many Passive Tags Can Standard Readers Interrogate at Once?

And is there any reader model that cannot interrogate multiple tags within the same field simultaneously?

—Name withheld

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First, I must note that there really is no such thing as a "standard" RFID reader. There are passive low-frequency (LF), high-frequency (HF) and ultrahigh-frequency (UHF) readers from numerous manufacturers, as well as a wide variety of active systems (Wi-Fi, ZigBee, Dash7 and more), and any answer must be qualified based on that fact.

Second, there is no RFID reader that can interrogate tags "at the same time." Interrogators read tags so rapidly that it may appear they are being read simultaneously. But in fact, the reader and tags are performing an elaborate communication protocol that allows the device to talk to each tag individually.

I have seen hundreds of UHF tags based on the EPC Gen 2 air-interface protocol being read within a second, but the actual number of tags that can be interrogated depends on a variety of factors. If you had 1,000 tags hanging in free air, not blocking one another's path to the reader, you might be able to read all of them within a second or two. However, if you were to put those same 1,000 tags on cases of glue in metallic tubes, you might not ever be able to read all 1,000, because the metal would likely block the tags on the interior cases from being interrogated. So to determine how many tags can be read within a second or two, you would first need to specify the context in which the tags would be read.

I am unaware of any readers that lack an anti-collision protocol allowing them to interrogate tags very quickly in succession, but there might be one out there somewhere.

—Mark Roberti, Founder and Editor, RFID Journal