It's been a long time since I've run out of SATA ports on a new build, but it still happens in home servers and NAS boxes. If you happen to have some M.2 SATA SSDs laying around and not enough sockets to throw them in, Alphacool could help you out. The Eisblock HDX5 SATA RAID controller isn't the sort of product we usually see from the German group, but it's actually named for the giant heatsink that covers both sides of the card.
The HDX5 is a PCIe 3.0 x4 expansion card with two M.2 sockets and two SATA ports on it. Despite the card's PCIe interface, the M.2 sockets on the HDX5 only support SATA M.2 drives. The on-board Marvell controller supports RAID 0 or RAID 1 modes across all SATA drives connected to the card, regardless if they're in a socket or plugged into one of the standard ports.
Alphacool says the HDX5's enormous heatsink should completely eliminate M.2 overheating issues, although we aren't aware of that being an issue with SATA SSDs. Still, if nothing else, the heatsink should protect the M.2 drives from damage while you're adding and removing other devices in your case. If you need to hook up four extra SATA devices to your PC, you can grab the Eisblock HDX5 from Alphacool's shop for €100 (about $97 without VAT).