I spend most of my time in northern Massachusetts and Southern New Hampshire. There are an inordinate number of vanity plates around here, particularly from the State of NH. I have one, actually. They’re cheap, and they’re easy to get. (You can order them online.) I’ve heard some people — particularly people from other regions of the country — make fun of people with vanity plates in NH. They’ve never seen so many in one place.
There’s a reason for that. New Hampshire has a relatively small population compared to Massachusetts and New York — 1.24 million, putting it at 41st in the Union — and where do most of these people live?*
In Southern New Hampshire. The northern regions are relatively unpopulated.
The Oxford English Dictionary places the number of words in the English language at ~500,000. Some of these are longer than 7 letters, but they can be abbreviated or shortened. Words can also be spelled by substituting numbers for letters as well. (l33t-speak) Zeroes cannot be used because they can be confused with the letter O.
So for simplicity’s sake, we’ll say there are 500,000 desirable letter-number combinations. (There are actually closer to 6.1 billion — 25^7 — but only a tiny fraction are desirable.) Figure that the 80% of the population is concentrated in the southernmost 50 miles of the state, and it’s easy to see why you see so many vanity plates in southern New Hampshire. It’s not that the absolute number of plates is higher than, say, New York, it’s just that they’re all in one place.
*For reference, Essex and Suffolk county (Boston and Metro North) have a combined population of ~1.3 million people.