I have a 4/12 pitch roof and my front porch has a 5 ft overhang... do I need support posts?. the porch is 18ft wide 5 ft overhang.

Not sure what size the tail end joist(s) are but even with a doubled 12, you would need one. You have a pretty good static load. I have a 40 ft porch that is 8 ft wide and the posts are 8 ft. apart.