Bonjour @julien.guerry ,
The number of points can be read a posteriori after processing Step 2 from the quality report.
The factor 4 or 16 is defined by Pix4D in order to decide how many additional points must be created. I remind you that the purpose of the densification is to densify the cloud of points generated during step 1, those we refer to as Automatic Tie Points. You can find additional information at https://support.pix4d.com/hc/en-us/articles/203269885-How-many-Points-are-generated-during-step-2-Point-Cloud-and-Mesh.
For your case, it is correct to say that one 3D point is computed every 4/(0.5) = 8 pixels of the original image. Could you point me to the place where it says that with the “Optimal” option a 3D point is created for each pixel of the image?