What does the little number in IIS Express system tray manager mean?

When opening the system tray adminstration tool for IIS Express there is a little number at the side of the site name (here the number 3):

Can anyone tell me, what that means?

That number does not mean anything, it is part of the site name. Here "...Web(3)" is site name. If you want you can rename it.

1. Open %userprofile%\documents\iisexpress\config\applicationhost.config
2. Search for 'Site(3)' and rename it