How bad is it to brush your teeth 10-20 times a day?

Excessive toothbrush. Brushing that many times a day is dangerous because it can wear off the enamel of your tooth. Toothpastes are abrasive to remove surface stains if you use them excesssively you wear through the hard enamel. You only need to brush twice in an effective way to fight decay and gum disease.
Why not cut back. 3x/day is sufficient...If you feel that you're having bad breath issues, there are products that can help. Also, be certain to floss to remove bacteria between teeth. Generally, brushing too much/frequently/hard can damage the enamel along the gumline.
Why do you need to? The question should be, how many times do I need to brush to effectively clean my teeth. The answer is at least twice a day. Brushing after meals can bring that up to 5 times a day. You don't gain anything by brushing more. If you use a hard-bristle brush you can harm your teeth by brushing too aggressively. If you brush gently, with a soft brush, you're probably not harming your teeth.