Does this M3 screw have 50 threads per inch?

M3 screws are metric: diameter 3mm, and 0.5mm threads = 2 threads /mm. As 1 inch = 25.4mm, you have 50.8 threads / inch

What is the length of these screws?

The length of these screws is 0.25 inches.

One package includes 50 screws.

Yes, these screws can be used for mounting motherboards.

Yes, these screws are compatible with optical drives.

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StarTech.com SCREWM3 PC Mounting Computer Screws M3 x 1/4in Long Standoff - 50 Pack

Keep stocked with M3 computer case screws for hardware installations
Works with most standard desktop computer cases
M3 screws / computer case screws / chassis screws / 50 x M3 / computer screws / computer repair screws / motherboard mount screws / floppy/CD-ROM/DVD-ROM drive screws
M3 thread type
0.25in length
Silver finish
50 screws per package
Works with many different types of M3 threaded computer hardware such as optical drives
Build computer systems or install new hardware