Well, that's a simple mathematical solution: 10^8 = 100,000,000 unique codes (including the "00000000"\) if you consider numbers 0 to 9 to be valid characters. If you include all the letters (e.g. a code could be "A11R30DF"\), that makes 2,821,109,907,456 unique codes. But then, you can have a mix of these (two first characters can include a letter, others don't, etc).

The minimum would be about 100 million codes.