I find that there are several commonly overlooked considerations when sizing a vSAN environment:

• It is not recommended to operate a vSAN environment at over 70% capacity
• If you use a resilience strategy of FTT=1, you should plan to perform a full evacuation during host maintenance or else you will be at risk of data loss due to drive failure during maintenance. Depending on your configuration and usage, the time required for a full evacuation can easily take 24 hours or more. In addition, a maintenance strategy of full evacuation requires you to leave one host’s worth of capacity empty.
• Because of these considerations, I recommend a resilience strategy of FTT=2. With this strategy you have the option of performing host maintenance using ensure-accessibility rather than full evacuation, which is much faster but is still resilient to one failure during maintenance.
• If you size your environment strictly to the minimum number of nodes for your configuration, then you will fail to create virtual machines or snapshots during host maintenance—including any snapshots used for backups or replication—unless you force provisioning of the object contrary to the storage policy. For this reason, you should consider provisioning at least one more host than is strictly required.

Many of these considerations are summarized in this helpful VMware blog post, which includes a helpful table documenting host minimums and RAID ratios: Adaptive RAID-5 Erasure Coding with the Express Storage Architecture in vSAN 8.

I’ve taken these considerations and created a vSAN sizer Excel workbook, to help both with planning and sizing a vSAN environment.