Are all of those PCs still in existence and still running MS-Excel (or the MS-Office suite) on them? Most licenses allow you to have some set number of machines that are currently operational running the software you've licensed. If any of those has died, had MS-Office uninstalled, or been taken out of commission and gathering dust somewhere you can request that Microsoft re-activate the number of licenses that match the number of "no longer operational" machines.
You could probably call Microsoft's Disability Services to get assistance with this if your situation matches what I've mentioned above.
Never underestimate the difficulty of changing false beliefs by facts.
~ Henry Rosovsky