After signing in to 10 accounts, Google shows this error message: "You are already signed in to the maximum number of accounts. If you want to use another account you must sign out of all Google Accounts then sign in to the account you want."
The percentage of Google users that have so many accounts must be quite small, but this feature would be really useful if it supported more Google services. Some Google users have different accounts for AdSense, AdWords, Blogger, App Engine etc.
{ Thanks, Q B. }