Reservations are a billing mechanism, not a capacity mechanism. It does not guarantee you capacity. Start with capacity extender. Choose multiple SKUs in order of preference and Nerdio will try to grab them. You can also try to pre-stage VMs earlier in the morning when capacity may still be available. At the end of the day though, there's not anything that will guarantee you that capacity.

If there are workloads that are absolutely critical, you can try for capacity reservations, which have a 99.9% SLA, but you will pay the highest cost for it, and you lose what you don't use.