Have a look at https://www.youtube.com/watch?v=RwCecNHTR54 and https://www.researchgate.net/post/ImageJ_FIJI-how_to_count_a_fraction_of_cells_which_are_positive_in_terms_of_a_certain_protein_expression

I think in ImageJ, ANALYZE panel ther are set of options that worthwhile giving them your attention such as "Label", "Mesurements", and "3D Objects Counter". In some versions of ImageJ , they are not available, but you can easily download then insert them to your ImageJ platform.

Alternatively, careful thresholding, also helo to segment objects in 2D image or 3D stack of Images.

Best wishes.