As mentioned above it's more about ROI than amount of domains sold although I think for confidence etc. selling at volume with ROI is always my ultimate aim.
In total have sold well over 100 domains but not sure exactly how many and I don't like selling cheap. Have sold at least 80+ this year so far but is partly skewed due to bulk chinese themed domain sales (4l.coms, 6n.coms etc.) in the $xx,xxx range. Sold other names for $x,xxx and low $xx,xxx each to end users/afternic and some names at auction this year for $xx, $xxx, $x,xxx each.
Factoring in the time spent is important as well and to make anything decent in this part or full time takes huge time and pretty significant financial resources IMO.