How many Ubuntu users are there worldwide ?

It is obviously not possible to provide solid data, since everyone can distribute Ubuntu for free (e.g. via a CD). Canonical (as one distributor) has therefore no way to know the exact number. (As a note: even companies like Microsoft might only know the number of legal owers of their software, but never the number of users).

What you can get are indicators of the number. You can estimate the percentage of Ubuntu users which are online with the default software configuration and multiply that by the number of registered website accesses that identify Ubuntu users, and multiply this by the percentage of those Ubuntu users who might have visited those websites.

Similar ways are applied in most estimates that you can read on somewhere on the internet. Often unique IP addresses are counted on security update repositories, but since I run an ubuntu server on strato, where all updates are always coming from strato servers, Canonical would never be able to include that server in any statistic. Also, since that server never browses on any websites, estimates from statcounter or similar will never track that installation. So again: no way to get to a solid number. Only estimates (and plenty of them)...

(I am very confident to say however, that we will have 200 million users in the next 4 years :-)

I think in most markets we're still around the 1-2% mark. People don't tend to like that stat. They want it to be 10% and try to swing sales stats around like they mean something, but I've made several logical arguments for ~1% on my blog.

Neither Canonical or Ubuntu counts its users. Downloads are not centralised by design so there's no way to count downloads. Because there aren't licenses there aren't sales to count and even if there were, we're dealing with free software. Downloads don't translate into installs or users. It could try a phone-home counter to do that but to but that would frankly be suicide (omgprivacy!), and there are still derivative users (Kubuntu, Lubuntu, Xubuntu, etc) who wouldn't be counted with an Ubuntu-only solution.

So while it's impossible to get exact numbers on these things, we can get some number from clients of various services. These give us a relative number (like 1%).

• Web browsers give out their OS when they visit a site. There are problems with this data but if you look at the stats for the right website, most of the biases pale away. Here's a maintained collection of statistics from browsers.
• Steam publishes hardware survey data which includes "OS Version". This is obviously geared to gaming so only really represents the percentage of Linux users in amongst Steam users.

According to the Ubuntu homepage at http://www.ubuntu.com/ there are at at least 20 million users that use Ubuntu every day. Las year Ubuntu estimated that there were around 12 million people that they "know" of. So its great to see such a big increase!