So you have: letter - digit - letter - digit - letter - digit .
The question doesn't say anything about restrictions on which letters can be used, or restrictions on repeating letters or digits within one postal code. So as far as we know, each letter can be any one of 26, and each digit can be any one of 10.
The total number of possibilities would be
(26·10·26) · (10·26·10) = 17,576,000 .
In the real world, though, (or at least in Canada), Postal codes don't include the letters D, F, I, O, Q or U, and the
first letter does not use W or Z. When you work it out with these restrictions, it means there's a theoretical limit of 7.2 million postal codes.
The practical limit is a bit lower, as Canada Post reserves some codes for special functions, such as for test or promotional purposes. One example is the code H0H 0H0 for Santa Claus ! Other special codes are for sorting mail
bound for destinations outside Canada.
At the present time, there are a little over 830,000 active postal codes. That's about 12% of the total possibilities, so there are still plenty of codes left for expansion.
