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Your problem is this bit:

      for(byte i=card_no*5;i<(card_no*5)+5;i++){
        crd_buf[i] = rfid.serNum[i];}

For the first card you're copying bytes 0-4 of the serNum to bytes 0-4 of crd_buf.

However for the second card you're copying bytes 5-9 of serNum to bytes 5-9 of crd_buf. That's correct for crd_buf but wrong for serNum.

The right way would be to add card_no*5 to i when creating the index:

      for(byte i=0; i<5; i++){
        crd_buf[(card_no*5) + i] = rfid.serNum[i];}

Or better to define your card buffer as a 2D array so you don't need to do that calculation:

byte crd_buf[CARDS][5];

for (i = 0; i < 5; i++) {
    crd_buf[card_no][i] = rfid.serNum[i];
}

Or simpler:

memcpy(crd_buf[card_no], rfid.serNum, 5);