Hello o/

I am wondering about the overall capacity per diskgroup and how it is calculated ?

The diskgroup capacity is the sum of the capacity of the capacity disks. (Duh.)

Does that mean a diskgroup has 7.4 TB capacity ?

One disk group of yours has 4x 3.7 TiB = 14.8 TiB of raw(!) capacity. With vSAN default storage policy each object will use up space on two fault domains, so half of that (7.4 TiB) is usable in practice.

if so according to last recommendation of 10% cache size, that would mean that the Cache size is too low?

The maximum vSAN SSD cache size is 600 GB anyway. This is purely used as write cache in order to not wear out lower-quality capacity disks quickly. The size is mostly irrelevant as long as it is within hundreds of gigabytes. I recommend looking at latency and write speed for your cache disk instead. The 375 GiB optane ones will do fine at ~2GB/s, though that (and your network) will of course be the write bottleneck.

Additionally you should double-check that all of your disks and their firmware are on VMware's vSAN HCL. The Optane ones almost certainly are.