Any idea where I can find that magical number?
I assume it is small number, but having something concrete would help.
Best to get it from your own web stats. I think AWStats has the ability to report that number. Otherwise you can get the number from w3schools, but the stats will be reflective of their visitors -- not yours. The same will be true of any site you pull the stats from. And, while you may be able to find a site that rolls up the stats from multiple sites, you're still dealing with a small subset of sites, and it's not going to be representative of your users.
Rambling about stats aside ... Could you explain the core issue a little more? Are you just talking about whether to use the HTML doctype? Or an HTML5 feature (or tag) or set of features (or tags) in particular?
I guess ... regardless of what your answers are, I would say this. Check your browser stats using AWStats, Google Analytics or some other stats app to determine the oldest browser using your site with a share greater than P%. (say .. 5%) Design your pages to be compatible with THAT browser. Add <script>'s to enhance the old-browser compatible page. The script can test the browser to see whether it supports enhancement X, Y, and Z and apply the enhancements if it does.
Know what I mean?
Design for a crippled browser. Use scripts to "upgrade" it dynamically, when possible.