Use as little of it as you can get away with!
This depends on the hardness of your water and the soil in your clothing.
If you live in Vancouver (some of the softest in the world) the soft water requires quite a bit less detergent than hard water yet the more soiled the clothes, the more detergent you need to clean them.
See the instructions on the detergent package for specific guidelines.
For Vancouverites we can use as little as 1/4 of what is mentioned on the package/scoop.
For most top loaders usually 1 tbsp-1/4 cup and not much more. If suds/foam stand a couple inches tall and/or cover all across the top of the water or all the way onto the floor, or worse, remain at the end of the cycle, you are using way too much detergent.
There is such a thing as too much. This can damage the machine by washing out the bearings.
Too much can leave you wearing the patch. Not a nicotine or birth control patch, but a detergent patch (head to toe). That can’t be good for your body.
If you, or anyone in your house has a rash or sensitive skin, reduce the amount of detergent used and/or use “extra rinse” feature. Wait up to a month for results. Switch brands if necessary. Look for a small dollop of bubbles on top of the water (~1-3 inches in size) this means you’ve used only a little too much.