to answer your question http://www.alexa.com/data/details/traffic_details?q=www.google.com&url=www.google.com
Alexa reports: Reach per million users: Today 255,500 this means that acording to alexa, of each million people surfing the internet, one fourth is going to google.
Alexa is a horrible tool for comparing lower rated sites, it is pretty much agreed that ratings as high as 1000 or so can be manipulated easily and you have to keep in mind that some infamous spyware sites have been ranked very high on Alexa ( I think Gator got as high as 5). Oh, the irony... So the true number of visitors to Google is probably only known to Google admins.
I should have been more specific. Alexa's data is only an statistic. Calculating the aproximate number of internet users and those who is its toolbar and then giving that aproximation. Much like polls of opinion, they give you only an idea of what's going on, but not a hard fact. thanks for pointing it.