The Intel® SSD 750 Series uses a PCIe* NVMe* 3.0 x4 interface, so it will reach its best performance in a PCIe Gen 3.0 slot with 4 lanes (x4).
Doing a quick check of the Asus* Sabertooth X99 Specifications, your motherboard uses this setup with a 40-Lane CPU:
3 x PCIe 3.0/2.0 x16 (x16, x16/x16, x16/x16/x8)
You might want to double check with Asus Support, however, it seems you would have room for two x16 cards (Video cards), and in the 3rd slot, you will have x8 capability. This would be fine since the 750 only requires four lanes (x4).
The mention of 18 channels in some online websites refers to the capabilities of the controller handling the NAND flash memory of the drive, this is an internal feature of the SSD and is not related to PCIe* lanes.