Hi.. plumbing code says fall of pipe must not be greater then size of trap.
So a 1-1/2 pipe/trap at 1/4 per foot fall needs a vent at 6 feet. Question if I run this pipe 20 feet, do I need more then one vent along the pipe? Every 6 feet?

No. Just one vent for each trap. I know that IPC allows for more distance for the trap to vent, but our local code only allows 3 1/2 feet for trap to vent distance. Mind you, these are maximum distances. You can put the vent connection closer, too. No less than 3 inches for a 1 1/2" trap to avoid crown venting the trap.