Tea contains caffeine which should be consumed in moderation.
Caffeine can make you jumpy and edgy.
Some studies suggest excessive tea drinking can cause brittle bones and teeth (as well as staining teeth).
20 seems excessive. Some days I may have 3-4 cups, others I wont bother having one at all. But, I know for some people a "brew" is very comforting and part of their routine, I know people who have a cup every hour, at least.