So here’s my recent experiences...
Adsense: I’m down to one responsive unit and one link unit with six links in it. Link units are strong as hell. The responsive unit is high on the page.
Media.net: three mid-article units. After the five days or so that optimization took, they have been consistent at about 2.25 to 2.50 total rpm. Not bad. Three units is their max. Less units does not result in higher RPMs per unit.
Third-party header bid: The less units, the better. I use one unit in a position right on the fold. Sky high rpm but more units rapidly leads to futility.
Total RPM from these three combined is about 7.00 to 8.00.