The solutions that are outlined in this reference architecture are based on a single indexer node. This node performs all indexing and backend searches for the stated data rate. The solutions are specified this way for efficiency. Spreading the indexing work over multiple nodes using clustering requires greater storage overhead but also adds capacity.
Purchasing a single, higher-cost indexer as opposed to multiple lower-cost indexers can result in a significant cost saving for the overall solution. Operational costs are lower for fewer nodes. A clustered solution adds high availability and may justify a higher cost.
If you require more cold storage, adding a NAS like Isilon, rather than increased indexing rate or search rate, is often more cost effective. Using a NAS solution for cold storage is appropriate because the performance requirements are lower than for hot and warm storage. Contact your Dell EMC representative to discuss sizing configurations details.