Since OD600nm is measuring scattering and not necessarily absorbance of light, the relationship between OD and cell count may be different depending on the culture media, the species of bacteria, and the spec you're using. To have a truly reliable handle of this, you'd need to prepare serial dilutions of a culture, measure their OD on the same spec, and then plate these and count CFU to calibrate your standard curve. See https://www.protocols.io/view/calibration-protocol-conversion-of-od600-to-colony-36wgq9e1xlk5/v1 for a great protocol.

As a general rule of thumb, you can roughly approximate that an OD600 = 1.0 is equal to about 8x10^8 E. coli cells. Remember that OD is a log scale, so that means an OD600 = 0.1 is about 8x10^7 cells. But, not all specs are the same and not all bacterial cells are the same size or scatter light in the same way.

Hello, for establishing relationship between OD and cell count you need to have culture suspension with known amount of microorganisms. For that make measurement OD and than grow on plate. First make suspension measure OD at 600nm than use same suspension to count the microorganisms by growing on plate, better to use 3 plates in each dilution.