What is the 12-bit BCD binary code for 53810? (u8q12) ?

This is weird. A BCD code allocates 4 bits (a nibble) for each digit. So a decimal number like 53810 has 5 digits, so would require 5*4 = 20 bits.

53810 = 0101 0011 1000 0001 0000

So, I don't see how you would fit that in 12 bits. What am I missing?