The halftone line count is a measurement of printing resolution, also known as line or screen frequency and lines per inch (LPI).
LPI is the resolution of a halftone image—the higher the LPI, the higher the density of dots. Thus, the higher the image resolution.
According to the American Screen Printing Association, common halftone line counts for garment screen printing are anywhere from 35–65 LPI.
When printing halftones, it's essential that the LPI and mesh counts correlate. A specific LPI will require a certain mesh count.
Get more details on mesh selection for halftones here.