How do I measure bacterial growth in agar dishes (either by cell mass or by cell count)?
The most simple way is seeding the plates with a suspension of bacteria ensuring that you spread the solution properly. Then you can count the number of colonies, wich would be equal to the number of single cells.

If you want to mesure the growth speed, usually it's simpler to just measure the diameter of the colonies, always ensuring you inoculate the plates with the same amount of inoculum.

Lastly, it's even easier to estimate the growth if your culture is un liquid media and you measure the optic density with an spectophotometer.