While this is not an answer to your question, may I ask why it is a problem that someone “known” how many sites you may have? (No one will actually be able to tell the number of sites you are tracking as there might always be a site that has n+1)
The reason I would prefer not to have someone see a specific number is client confidence.
If I give a website an site ID of 3, they always think well that means I have only 2 clients before them.
It may sound like a petty reason but every little helps
My primary desire for a feature like this is so I could give it a unique code that fits with my standards as well. For example, Site ID -> 1 to Site ID -> ABCDEFGHIJK. This would allow me to script my tracking code generation as it is a known value whilst the Numeric site ID number has no reference to my other standards.
I would also like to have a custom site ID.
Currently it is possible to find out all existing sites (by id) and create spam and other malicious things to that sites. @halfdan piwik will respond with a 404 in the tracking request when a site doesn’t exist, so you can find out exactly how many pages there are.