As it comes out of the box, you can use up to 4 SATA devices (SSDs, mechanical drives, optical drives)
The motherboard also has 3 pci-e slots. In each of these slots, you could insert a pci-e to M.2 adapter card, which would allow you to install a NVME (not SATA) M.2 drive on the adapter card. It will appear as a regular SSD, but you probably won't be able to boot from it as the BIOS doesn't know how to do that.
So if you use an integrated video card, you have the pci-e x16 slot available. A pci-e to M.2 adapter card will give you 4 pci-e 3.0 lanes so you'll get up to 4 GB/s read and write speeds.
The pci-e x1 slots are pci-e 2.0 , so you'll get a maximum 500 MB/s read or write speed, a bit less than the maximum SATA can do.
Here's a pci-e x4 to M.2 card - you could use this in the pci-e x16 slot: https://www.ebay.com/itm/M-2-NGFF-to-Computer-SATA-Dual-SSD-PCI-PCIe-x4-x8-x16-NVMe-Express-Adapter-Card/202760892790
You'll notice it has two M.2 connectors. One is NVME only (goes to pci-e slot), the other is SATA only - you can connect a SATA cable between the SATA connector on the adapter and the one on your motherboard, and this way you can use a SATA M.2 drive in your PC.
You can't use two NVME only SSDs with this adapter card, only one.
And here's a pci-e x1 to M.2 (nvme) adapter card :
So with these cards, you could install up to 7 devices (if you plan to use integrated graphics) or 6 devices with a video card.
Another option is to use HBA adapter cards, basically they're SATA controllers which create additional SATA / SAS ports. HBA simply means the cards don't offer any RAID functionality, they just make the SATA devices available as independent devices.
Here's some examples :
4 sata ports : https://www.ebay.com/itm/IBM-LSI-SAS9212-4i4e-6G-SATA-SAS-Host-Bus-Adapter-HBA-46C8935-46C8934-46M0907/192562587005
cards with 2 SAS connectors - you have to use SAS -> 4xSATA breakout cables which are easy to find... 2 SAS -> 8 SATA
card + 2 sas->4sata cables: https://www.ebay.com/itm/LSI-SAS-9210-8i-8-port-6Gb-s-PCIe-x8-HBA-RAID-SAS-SFF-8087-to-4x-SATA-Cable/133137220890
just card : https://www.ebay.com/itm/LSI-SAS-9210-8i-8-port-6Gb-s-PCIe-x8-HBA-RAID-SATA-Controller-card-M1015-9211-8I/142676059703?
just card : https://www.ebay.com/itm/LSI-SAS-9210-8i-8-port-6Gb-s-PCIe-x8-HBA-RAID-SATA-Controller-card-M1015-9211-8I/142676059703
just card : https://www.ebay.com/itm/LSI-SAS2308-8i-9217-8i-6Gbps-8Ports-HBA-PCI-E-Server-SATA-SAS-Controller-Card/132059450823
just card : https://www.ebay.com/itm/Dell-Perc-H310-SATA-SAS-HBA-Controller-RAID-6Gbps-PCIe-x8-LSI-9240-8i-M1015/192120863360
just card : https://www.ebay.com/itm/LSI-SAS2008-8I-SATA-9211-8i-6Gbps-8-Ports-HBA-PCI-E-RAID-Controller-Card/131856071144
the SFF-8087 to 4x SATA cables : https://www.ebay.com/itm/50CM-Internal-Mini-SAS-SFF-8087-36P-to-4-x-SATA-7-Pin-External-Connector-Ca-W2R5/123883948349
You can buy HBA cards from eBay and other places for as low as 20-30$ and they're typically pci-e x4 or pci-e x8 but pci-e is designed in such a way that a card with a bigger number of lanes will work in a slot with fewer lanes, so these adapter cards will work even if you install them in the pci-e x1 slots.
If you can't physically insert them in the slot, you can buy riser cables / cards which create a pci-e x16 slot: https://www.ebay.com/itm/Flexible-PCI-E-PCI-Express-cable-X1-TO-X16-Riser-Card-Extender/162820955963
So if you use two of these cards, one in each x1 slot, you could have up to 20 SATA devices in your PC ( 2 x 8 sata from adapter cards, 4 sata from motherboard)