It would literally be impossible to accurately determine how many studios are using Blender.  There aren't many who are admitting to it, that's for sure. The reason for that is laid out quite intelligently in this article by Sean Kennedy, about Blender's use in Hollywood:

http://blenderdiplom.com/en/interviews/548-interview-sean-kennedy-on-blender-in-hollywood.html

As Sean puts it, you have studios which invest big money into developing their own proprietary software, or extensions to existing software, and those studios like to talk about that when asked how they accomplished certain shots.  Deservedly so.  So much of the ground breaking technology brought to VFX is the result of countless, uncredited nights of hard work by largely unrecognized but extraordinarily talented individuals.  While Blender is being used in some studios, it's usually a minor or very specialized role somewhere in the pipeline, where a particular need is required, but expense doesn't justify paying the TD or programmers to create a customized extension to the current toolsets.  This is where the open source nature of Blender comes in very handy in saving resources or expenses.

One major misconception also is that all professional level studios use all proprietary software.  This is completely false.  Many many high level VFX and Game studios are using commercial grade software, usually in a highly customized and extended modular form.  Some studios are also being indirectly assisted by major software companies, and given unprecidented access to  in-house technical support, and privatized or micro-focused development, in exchange for crediting their products in behind the scenes interviews, and special features.

As for hard numbers, the most we could glean is that Autodesk is a major powerhouse in the world of VFX and 3D.  Let's look at numbers provided by Forbes about the company:

http://www.forbes.com/sites/shelisrael/2013/01/23/1785/

Over 100 million users have downloaded their software, or accessed the company's online properties.  As of 2013, this number was growing at the rate of 15 million users per month.  Even if just 1 percent of that number translates to active registered users, per package, per month, that's a very respectable and hugely influential number.  The price point of their software also means an incredible flow of monetary resources, which translates to access to some of the best programmers and developers money can buy.  Regardless of your personal opinion of them, it's clearly evident that they have the financial backing to do some incredible things.  It's fair to speculate that no software would be generating figures like this if they weren't proven effective, and worth the investment.

It's fairly evident that for independent artists, Blender is probably the best investment one could make, but large companies and studios have reasons to use other resources.

- Daniel Kramer
Modeling Supervisor/Senior 3D Generalist
Intrigue Studios, EncoreFX