I was wondering how I can call multiple external css files at once instead of calling each individually. I want to do that to avoid the request number and minimize it to one
Even if you can minimize your html code to only one line, the client has to load them all individually 'cause they still are in different files on the server.
If you want to combine the Styles into one file just do it manually or on server-side.
Furthermore you could minify your CSS with an online tool to make the file's size smaller.