Also Marissa and I tried to calculate how many license plates are of the form "XXX YYY". (Minnesota uses 3 letters + 3 digits, or the reverse which can be ignored.) Our back-of-the-envelope estimate while driving was about 3 per 100,000.
There are about 26^3 different letter combinations. Some are prohibited--- we don't know how many, but if it is a list rather than a ruleset then the number is probably small. There are about 1000 different numbers, but 000 is prohibited. So that gives about 26*9 trip-trip combinations out of 26^3*10^3 = one in every 75,000 plates, about 13 per 100,000.