There were 2 discs in my package.
Hi Lo, The largest pack size for the Diablo 4.5" 0.040" metal cutoff disc is a 25 pack. This can be found in some Home depot stores and on home depot.com. Here is the link https://www.homedepot.com/p/Diablo-4-1-2-in-x-0-040-in-x-7-8-in-Thin-Kerf-Metal-Cut-Off-Disc-25-Pack-DBD045040125F/205458812
I have used this disc for 2” x 2” aluminum and it worked great!
I don't believe it is safe to use this blade with a rotary tool capable of such high RPM, the blade is not rated for that speed.
Search the part number for your grinder. It'll tell you what diameter disc that unit takes. In this instance, your particular grinder used 4 1/2" diameter discs.
I beams are usually 1/4 " thick at least. These blades will cut it, but will take a long time. If you use a acetylene torch or plasma torch, it will take a lot less time. Also a skill saw with a steel cutting blade will be better also. Steel cut blade is usually about $45-60.
You best answer to your question would be, ask a professional at Home Depot they would know the best.
This disc is not meant for that purpose of removing rust. There are other blades more appropriate for grinding and sanding that will work better.
I've easily cut various grades of hardened steel that a hacksaw could barely scratch, so I don't think it will be a problem.