How many rats are best (for the rats)?

I know that owning one rat is pretty damn evil, and people say that you need at least two rats so they don't get depressed. But is two rats actually ideal for the rats' happiness, or is that just the bare minimum? Is there a 'golden number' for how many rats to own?

Also, let me know if I should change the post flair or take this post down. I'm pretty new here