HI Wim, by default adding a point on a surface adds both a row and a column of points - if you set the "Direction" option to add it only in the U or V direction instead of both then it will add less points since they will be added in one direction only. But the degree of the surface will be increased to degree 3 so the plane turns into 4 points in each direction as its base form, then any additional points will get added on to that.

If you want to more directly control the control point layout of the plane, try drawing a control point curve and then extruding it, that will give you just 2 points in the extrude direction and as many points as you create on the curve in the curve direction. If you want more than 2 points in the other direction then you can use 2 control point curves at 90 degrees to one another and then use Extrude with the "Set path" option as described here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=817.4

Hope that helps,
- Michael