Just for giggles Rick.
How many licenses are there?
I honestly don't know if anyone has ever counted over the years. I know that when I was involved the thought was never how many have we sold, but how many more people can we get involved and sell to. Knowing Rick I'll bet ... this unopened can of pop on my desk

that he still thinks the same way to this day. You never want to rest on your laurels and you always want to be moving forward.
You can use google to get a pretty decent idea of the # of sites running different scripts, we did it over at
UBBDev a year or 3 ago

You choose something like:
"Powered by
UBB.threads" and include a word that refers to a page that is only used once in the forum, like faq.html or faq.php
Not very scientific, but gives an approximation. Includes pirated software but not those on intranets or those who never installed/pulled the site down after buying.