Hi @Tianyu Sun-MSFT ,
Thank you for the reply, when it comes to the first option you proposed (ItemGroup), the problem is that in the Reference list I get the names of other Projects, not the names of the files. When it comes to the second option, the Output window, my project has a large number of files so it would be difficult to go trough the Output window and count them.
My solution or my idea is to use "Find And Replace" option, for the symbol which I want to search I can enter whatever and then I need to set "Include external items" and to press "Find All". In the output window I will get the number of files which are searched and that might be my answer.