The Image Volume Fractionator probe lets you sample a 3D region of interest and estimate the total number of particles.
IVF workflow guide
If you select Load subject data from existing file, typically directs you to the Count Objects step. Because the workflow is streamlined, only the relevant steps are displayed and they are re-numbered accordingly (e.g., Step 10 in the complete workflow might be referred to as Step 4 in the streamlined workflow).