The problem with trying to detect multiple RFID tags is that there's no way to select which tag is going to respond if you just throw them into a box. When the reader's antenna activates, any RFID tag within range will activate and broadcast its tag data. Whichever one is picked up by the RFID reader is up to the positioning and chance. Even if you have multiple readers, you have no guarantees that each will read a different tag or that the tags won't interfere with one another.