I have a fence 7' tall with 4x4 posts every 7'6".
The horizontals are 2x4 on 3' centres.
Safest approach is horizontals on 3' centres.

If you want the wood to look good for a long time use stainless steel screws as they will not rust and cause bleeding around the screw holes. As well when it comes time to replace any uprights all one need do is back out the screws.

P D Q
"Matt" wrote in message oups.com...
I am in the process of putting up a wooden privacy fence. All the
posts are set and am at the point of fastening the 2x4s in between.
My question now is, do I put in 2 or 3 horizontal 2x4s. Will using
two do the trick or is it necessary to use three? Thanks for any
advice.

Matt