[Mark Sir Link]

assuming you convert the population to base 26, you have the worst case number of license plates, 26^number of digits of population in base 26

you ought to be able to work linearly backward from that point to determine the best combination of numbers and letters, knowing that 26^digit places * 10^digit places will yield the total combinations for either.

Clearly the digit places of either of those should never exceed or equal the digits of the worst case scenario, since from the beginning you know the worst case would be 26^digit places of population in base 26, and can only improve from there, (IE start at places - 1 going backwards for letters, and 1 moving upwards for numbers)