I think, concentration varies from fungal species to species and even among the strain of fungi. Some fungi may cause severe post harvest disease, hence the concentration needed for inoculation to fruit might be low. I think, spore concentration is very important and you can calculate using haemocytometer. 

If you are inoculating fruits with B. cinerea the concentration of spore suspension needs to be adjusted to under optical microscope using hemacytometer. Concentration also depend on hosts . As Anum mentioned 10 ul spore suspension can be used to inoculate tomato and apple fruits.  

Hello, when you prepare spore suspension, preferably in water containing 0.05% Tween-20. Estimate the spores in this suspension by  haemocytometer or through McFarland and adjust  to the required concentration ((1 x 10 (6) spores/mL)

Good luck