RFID reader code to identify more than one tag
here's how I'd do it ... allowing for 4, 7 and 10 byte uid's (which are all possible apparently)

struct tagInfo {
  byte len;
  byte uid[10];
};
struct tagInfo tag[] = { // add your tags here
  // the len is 1, 2 or 3, for lengths 4, 7, and 10
  // you could just use 4, 7, 10 here and not bother
  // with the len * 3 + 1 calculation later
  // the choice is yours, really
  {1, {0x01, 0x02, 0x03, 0x04}},
  {1, {0x02, 0x03, 0x04, 0x05}},
  {2, {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07}},
  {1, {0x02, 0x02, 0x02, 0x02}}
};
const int numtags = sizeof(tag)/sizeof(struct tagInfo);


Then, your loop

void loop() {
  if (!mfrc522.PICC_IsNewCardPresent()) { return; }
  if (!mfrc522.PICC_ReadCardSerial()) { return; }
  int i; // declare before the loop because we want to check the value after the loop ends
  for (i = 0; i &lt; numtags; i++) {
    byte len = tag[i].len * 3 + 1;
    if(len == mfrc522.uid.size) { // 4, 7, 10?
      if(!memcmp(mfrc522.uid.uidByte, tag[i].uid, len)) {
        break; // found an exact match so stop the loop
      }
    }
  }
  if (i == NUMTAGS) { return; } // no matches
  // all good here - run the rest of your code
}