You will commonly see a letter accompanying the mesh count e.g. "T" (regular), "S" (Small) or "HD" (heavy duty) with "T" being the most commonly used designation. Here are some sample mesh counts 30, 60, 83, 109, 140, 162, 200, 255, and 305. There are of course, lower and higher sizes available, but these are the most common and readily available from screen printing supply vendors.
MESH COUNT = INK DEPOSIT
The most important thing you need to know about mesh count is that the amount of ink that will be deposited on the substrate will be determined primarily by the mesh count (other factors include stroke angle, ink viscosity and stroke technique).
24 MESH VS. 305 MESH
Said another way, the lower the number in the mesh count, the more ink that will be deposited on the substrate ("substrate" is the technical name for the item that is being printed.)
As an example, an extremely low mesh count is 24. You would typically use 24 mesh use if you are printing with glitter ink, which has large flakes of shiny, plastic material that have to pass through the mesh. As a result, the mesh must have a relatively large opening. So you can understand how big the openings are on 24 mesh, imagine what the screen material in the screen door of your house looks like–you can actually see the openings, as they are very visible to the eye.
By contrast, if you a printing a photographic image that has a lot of detail and consisting of halftone dots, you would use a mesh count is that is relatively high, e.g. a 305 mesh. A 305 mesh is a very fine mesh and requires high-powered magnification to actually see the mesh openings.
One more thing that you need to know about mesh is that it comes in different colors. Typically mesh counts of 162 and down are usually available in white only and mesh counts of 162 and above can be purchased in colors of yellow, gold, orange, and red. Higher mesh count fabric color is dyed so that when you're exposing a high mesh count screen, the light will not scatter as much as it would with white fabric, this factor having some importance when exposing critical images including halftones. Why there are so many colors to choose from is a mystery, but gold seems to be most common “colored” mesh available from most screen printing supply vendors.
The general rule used in our shop was to order gold colored mesh for all mesh counts of 200 and up.