Almost always a sample for counting must be diluted because the counting procedure would be overwhelmed.  Consider these sketches:

The counting chamber is placed under the microscope and observed at the highest magnification.  There is an etched section in a well.  The sample shown in green is placed on the etched region and squished down with a cover slip.  An ordinary cover slip would flex because of capillary forces, so a reinforcing rectangle is cemented to the top of the cover slip.  The dimensions define a precise volume over the etched area (shown too big in this sketch). The number of organisms in the grid is multiplied by the dilution to get the number in the original sample.

The Petri dish count depends on dilution of the sample to the point where there are enough colonies to be statistically significant but not so many that they overgrow each other. This count takes time for the individual cells to grow into visible colonies.