There are three common thread types for rack screws: 10-32, 12-24 and M6.
The legacy Ruckus ZD500 used Rack screws, 2 #8-32 x 3/4.
If you're mounting in a standard rack, I would go with the 10-32 screws as they would be the most common, and original rack screw size. Depending on your locale, the sizes or types may be different based on the kind of rack you wish to install this in.
If you're still curious and would like to do additional research, I would recommend checking out the following resource.https://www.server-racks.com/rack-screws-10-32-12-24-m6.html
Best of luck!