Hi, why you don't make it 8? There seems to be enough space on the left and Right to add another slot on each side. The consequences are roughly the same for someone wanting to add the cage: if only one SFF-8643 connector shall be used, 4 drives are usable (if now the connectors are used on a 4-2 basis) and 4 will remain unusable (instead of two). But if someone wants to use all connectors. two additional drives can be attached with the same amount of connectors.
Btw. my Intention was to have 8 carriers instead of 4
Anyway - if the m.2 Slots are to be NVMe then 6 will fit (makes it perfect for EPYC).