How do I find out the W3SVC number of a given IIS7.5 website?

The numbers on the folders correspond to the Site ID of each specific site in IIS. If you go into Internet Manager you can see the Site ID's by clicking on the Sites node in the navigation pane.

So, if a site has ID 1 its log folder name is W3SVC1, ID2 = W3SVC2, etc.

You could also review %WinDir%\System32\Inetsrv\Config\applicationHost.Config, which contains information about all of the sites. It is XML format. You'll want to look for <site> nodes in the XML which contain an id attribute. This is the aforementioned Site ID of that specific site and will align with the number in the log folder.

<site name="Default Web Site" id="1">