Linux is freely available, and no one is required to register their copies with any central authority, so it is difficult to know how many people use Linux. Several businesses now survive solely on selling and supporting Linux (and relatively few Linux users purchase products from those businesses), and the Linux newsgroups are some of the most heavily read on the internet, so the number is likely in the millions, but firm numbers are hard to come by.
However, one brave soul, Harald T. Alvestrand <Harald.T.Alvestrand@uninett.no>, has decided to try. If you are willing to be counted as a Linux user, please use the web forms available at http://counter.li.org/ Alternatively, you can send a message to email@example.com with one of the following subjects: `I use Linux at home', `I use Linux at work', or `I use Linux at home and at work'. He will also accept `third-party' registrations; ask him for details.
He posts his counts to news:comp.os.linux.misc each month; they are also available from http://counter.li.org/.