1. In an All Flash configuration you have one "Buffer" Device per Diskgroup and up to 7 capacity drives. Normaly its not from the same type as your capacity SSDs because you have a much higher Write for that disk and also the max. usable Buffer space is ~800GB?
2. The Buffer, as same as the Cache device in Hybrid configurations doesnt count when it comes to capacity. So the RAW capacity is only based on the number of your capacity disks. The usable space depends on RAID/ErasureEncoding Mode and FTT Policy.
IIRC there is a vSAN Sizer available from VMware.
Its worth noting that while there is a 600GB logical limit for a write buffer in All Flash, that doesnt mean that the same 600GB gets used over and over again, promptly wearing out your SSD. The extra capacity is used accross the disk to increase the endurance of the device which is one of the reasons why ~10% is still recommended even when it would result in a disk larger than 600gb