Two is about right. Leave one (the dimmer one) on all the time, put the other in a flash mode. Leaving a tail light on flash all the time can mess with a driver's depth perception, so leaving one on constantly helps alleviate that.
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I stop for people / whose right of way I honor / but not for no one.
Originally Posted by
bragi 
"However, it's never a good idea to overgeneralize."