One of the places I worked that was on-prem enforced a "standard hardware profile" where the servers were all nearly the same except things that could be changed in house (like RAM sticks). When they ordered hardware, they'd order like 5% or 10% more than we thought we'd need to keep an "emergency pool".
If you ended up needing more hardware than you thought and could justify why you needed it right now, they'd dip into that pool to give you more hardware on a pretty rapid schedule.
It cost slightly more, but was invaluable. Need double the hardware for a week to do a migration? No problem. Product more popular than you thought? No problem.