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
Two options.
1. Css @import
@import url(stylesheet.css);
See documentation.
NOTE!: The browser actually does make multiple calls here, it is just hidden from the HTML
2. PHP
Use minify to shrink, cache, and combine your CSS/JS files.
Here is a minify tutorial
Here is a list of more compression tools.