I recently built a Celeron J4105 system for a NAS/server/HTPC, in a compact Fractal Node 304 case. The ASRock J4105-ITX has four SATA 3 ports and a PCIe slot,

I recently built a Celeron J4105 system for a NAS/server/HTPC, in a compact Fractal Node 304 case. The ASRock J4105-ITX has four SATA 3 ports and a PCIe slot, to which I added a 2-port SATA 3.0 controller. Total six disks, which fits in the very compact case. I run a 120GB SSD for the system, and a random assortment of 2-3TB disks for storage, in a Btrfs RAID1 pool. All in, about 2/3rds the price I would have paid for a 4-disk Synology NAS. You can get micro-ATX boards with 6-10 SATA 3 ports, and PCIe board with basically as many SATA ports as you're willing to pay for.