whats max dist of posts for 4x6 beam with double 2x4 plate on top

Based on a 16"o.c. joist or rafter load above, 3-1/2' to 4'. A "beam" is usually comprised of 2 sticks rather than just one and yes 2 is better than 1. See this very useful Span chart -

- Your local Building Codes may be different and Building Codes are just the Minimum Efforts, they are not the best and they are not a guarantee of no future problems nor universal use for any load. A Hot Tub, Bathtub, Full Tile job, Piano, Waterbed, Woodworking Shop Equipment, High Snow-load, etc. will require additional material and efforts.