Welcome! The SAS extenders are nice, but more expensive and hard to find. If you are interested in going the pure SATA route, here is a solution that gets you 30 SATA drives per 4x PCI-e slot!
You need one of these: Amazon.com: Ableconn PEX-SA156 6-Port SATA 6G PCI Express x4 Host Adapter Card - AHCI 6Gbps SATA III Port-Multiplier PCIe 3.0 4-Lane Low Profile Controller Card (ASMedia ASM1166): Computers & Accessories
And six of these: Amazon.com: SaiDian 1 Pcs SATA 1 to 5 Port Converter Expansion Card SATA Port Multiplier Hard Drive HDD SSD Accessory: Computers & Accessories
Here is the newer 6g version, but it is more expensive. Amazon.com: GYF Controller Card Motherboard Sata Expansion Card 1 to 5 Port Sata3.0 6Gbps Multiplier Sata Port Riser Card Adapter for Comp: Electronics
I’m using the older one above and it is still plenty fast enough for Chia farming even maxed out with all 30 drives.
You can find these components a lot cheaper on eBay and Aliexpress of course, but they take longer to ship from China and the return policy is…questionable. Amazon is more expensive but easy to send back if it doesn’t work for you or you get a bad one.
Hope that helps!