I don't know how they keep track, but apparently somebody keeps track:

English contains more words than any other language on the planet and added its millionth word early Wednesday, according to the Global Language Monitor, a Web site that uses a math formula to estimate how often words are created.

The site estimates the millionth English word, "Web 2.0" was added to the language Wednesday at 5:22 a.m. ET. The term refers to the second, more social generation of the Internet.

The site says more than 14 words are added to English every day, at the current rate.

"Web 2.0" isn't really a word, though.  It's a phrase.