That is the user ID number for the different accounts. Depending on how many accounts you have set up in your Steam client, you will see one or more folders with a number in place of its name. Once you’ve found Steam’s installation directory, do the following to find the screenshot you are looking for. If you've installed Steam somewhere else, you'll need to navigate to the Steam folder wherever you installed it.
On a Linux system, the default is ~/.local/share/Steam.
