not many people test that afaik bur 26 did work here https://xpenology.com/forum/topic/8057-physical-drive-limits-of-an-emulated-synology-machine/?do=findComment&comment=78160   no any extranel (usb or sata) can't be made into a raid in DSM imho the limit is not hard at  just single digit sdX, i've had working config with a asm1166 where the controller "blocked" 32 ports and the disks after that got sdXX and where part of a max 16 disk array (918+ has 16 as default), as