How many clock impulses can a passive RFID get in 1 radio signal?Does it even have a CPU?
Passive RFID tags just react to the right RF field.  If you don't want collisions, it's up to you to make sure only one at a time is within the near field of the antenna.

Fortunately for you, between the extent of the near field, and the distance at which enough power is transmitted to run the tag, the useful volume around a RFID reader is rather small.  For most normal use cases, it's not too hard to isolate individual tags to read them.