If you have a relay that is on dedicated hardware, then generally the only limitation is the number of simultaneous TCP connections that the OS can handle and not the hardware, especially if using SSD storage. The other bottleneck could be the network card if it only has 1 gig and you have tons of downloads going at once across the LAN, but that can be solved with a 10gig NIC.
Relays don’t tend to need a lot of RAM or CPU. I think 4 cores and 8GB of RAM is more than needed in most cases. A lot of BigFix processing tends to be single threaded so you are usually better off with fewer but faster CPU cores than many slower cores.
I like to see fairly large relay caches for top level relays and relays behind slower WAN links. I also think that consumer level SSDs like the Samsung 850 Pro work well since the caches tend to be write once read many. You can get a good size SSD these days for fairly low cost, sometimes cheaper than 15k SCSI disks.