I use an LSI 9211-8i card in my NAS. That one has 2 SAS slots, which in combination with a pair of SAS -> 4x SATA cables allows me to connect 8 drives.
IIRC you can use further splitters to connect up to 128 drives to a single SAS port. So that single card can handle up to 256 HDDs. I reckon that should suffice for most people.
Before buying one, keep in mind that they can be flashed as a RAID card or as an HBA. Which one you should do, will depend on the OS you'll be running on that machine. FreeNAS for example works best if it can do its own software raid on the drives, so that one should be an HBA. Windows on the other hand will work best if the card does all the RAID stuff on its own.