The r5b.12xlarge pairs 48 vCPU with 384 GiB of RAM, the 8.00 GiB-per-vCPU ratio that marks a memory-optimized instance. In the name, "r" stands for RAM, "5" is the generation, "b" marks block-storage optimisation and "12xlarge" is the size within the r5b family. It runs on Intel Xeon Platinum 8259 (Cascade Lake) at 3.1 GHz, and AWS introduced it in December 2020. AWS has since shipped r6i, r7i, r8i and r9g; r6i.12xlarge is the direct replacement at this size. Spot's node ladder tops out below this size, so r5b.12xlarge becomes 3 × mh.vs1.2xlarge (48 vCPU, 384 GiB combined), $3.58/hr on AWS in US East (N. Virginia), from $0.063/hr on Rackspace Spot. Spot delivers compute as managed Kubernetes, so a node pool of that size is the native shape of the answer rather than a workaround — provided the workload scales horizontally.
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r5b.12xlarge gives you 48 vCPU and 384 GiB in a single machine. The largest comparable Spot node, mh.vs1.2xlarge, is 16 vCPU and 128 GiB, so the equivalent here is 3 nodes rather than one.
Spot runs as managed Kubernetes, so a 3-node pool is the normal way to run things rather than a workaround. If your workload already sits behind a load balancer, a queue or a scheduler, it spreads across them without changes. If it needs all 48 cores and 384 GiB in one process, it does not.
