They can literally be about anything you want them to be that shows off your writing ability. So if you're a generalist I'd choose 3-5 samples that are widely different topics to show that you really are versatile and you can talk about a wide variety of things.

If you truly have no idea what to write on samples, here are a few prompts, write a 750 words article about the pros and cons of 4K cable boxes, or 5 reasons that Tik Tok is better than Instagram for creators. 

One thing I want to mention here is the absolute most valuable, best skill you can learn as a freelance writer is to quickly learn a piece of information and be able to digest it and write a coherent article about it quickly. If you can do that you will be one of the most in-demand freelancers out there.