Even on-demand isn't guaranteed capacity. There have been a handful of times in my career where I've tried to spin up a lot more instances and got met with an out of capacity error. There's a special kind of reserved instance for guaranteed capacity.
You have the option to get On-Demand Capacity Reservations now which are separate from RIs. Capacity reservations don't require 1- or 3-year commitments like RIs do. This can be used if you want to get your savings through Regional RIs or Savings Plans instead.
A few years ago we ran builds on m5d instances (the ones with local nvme) and they'd pretty regularly run out of capacity in certain AZs (although it was usually only a single AZ and there'd be a slight delay before the ASG grabbed one from a different AZ)
Exactly this. Our group regularly maxes out instance types in a region for some of our simulations. Have to overflow the requests over to other regions in those cases. Unfortunately the load is not consistent enough to pay for the reserved capacity.
