I know that it would be better to stain cell nuclei with DAPI. However, when I take the images, I have a strong background of fluorescence, probably due to the use of cotton swab tfor remove cells in the upper nside of the transwell.
If there are a few nuclei per image or if this is a once-off analysis, I would agree with Moumita, use the Cell Counter plugin. It assigns a marker on the particle/nuclei counted which minimizes the error associated with the analysis.
When able to physically separate the migrated cells from less migrated cells (as you have done using a transwell type assay) then simply extract the crystal violet and quantify using spec. Alternatively PicoGreen would be a more direct measure (based on dsDNA extracted. Obviously do this after removing the cells on upper surface with cotton bud.