Well it depends on the brand and the line of paint. Check the label. There's enamel (which you use paint thinner for -- not "brush cleaner"), and then there's acryllic, which you use acryllic-based thinner (some folks swear by Windex, some use future to thin it out).
You don't want to mix (for example) acryllic with paint thinner. They're incompatible.
If your paint is really really thick, like you dip the tip of your brush into it and a large irregular blob comes up, you only need a small amount of paint. Put that into the mixing dimple, and drip some thinner in to the same thing. Use the brush to smear the paint blob around inside the puddle of thinner. You must use your best judgement to determine if you need more thinner or not. The thicker the paint the more the ratio of thinner to paint must be.
However don't use too much thinner or there's just not enough paint to ge the job done.
Also if you use TOO much thinner you run the problem of affecting previous coats of paint (if there are any where you are painting). That doesn't happen often but it's happened for me before.
EDIT I might have misread your post. You mean in the 6-bottle packs, there's a 7th bottle that's clear, right?
If that's what you mean, I believe that is brush cleaner, and not thinner.
