Difficult to say without knowing how heavy the fence needs to be and what dimensions you want for the supporting parts. Should there be nail battens between the posts, or just posts with horizontal paneling? What dimension for the nail battens (if you're using them)? What dimension for horizontal paneling (if that's all you're using)? Noise barriers that the Swedish Transport Administration builds typically have large structures as framework, and then 3.5 meters between each foundation is sufficient, but those fences are often more than twice the height of what you want to build. It could be a benchmark anyway.
My neighbor and I made a fence between us. We placed the posts 2 meters apart, but it wasn't enough; the fence sways. It might be because we chose adjustable post bases, which become very wobbly. Construction mistake that I have to take responsibility for 
I built an almost 80 running meters 2.10 high with 95x95 posts and 3x45x95 nailing battens with 145x21 paneling and batten, placed 2m between the posts and used anchor bolts - it's sturdy enough to climb onto.
My tip is good anchor bolts and templates for quickly setting them; just use half the paper tube if you're drilling as it's easier to adjust and not so hard to get down.
My tip is good anchor bolts and templates for quickly setting them; just use half the paper tube if you're drilling as it's easier to adjust and not so hard to get down.