motherboard with a lot of pcie slots
PCI Express - Wikipedia
Asus announces B250 Mining Expert motherboard with 19 PCIe slots and triple 24-pin power connectors.
m.2 slot pcie - Suspended Domain
Similar to the PCIe expansion slots of your motherboard, the number of PCIe M.2 slots it will offer also depends on the number of PCIe lanes your motherboard supports.
Motherboards Explained: The AAFP - Overclockers UK
To use a PCIe SSD, the motherboard must have available PCIe slots and, for M.2 PCIe SSDs, an M.2 slot that supports PCIe.