M.2 PCIe x2 and SATA connections and key/notch with SATA/PCIe x4 SSDs
A card that has B and M notches is generally SATA, although it could be NVMe x 2 if the CONFIGx pins are set to that mode.

An M-notch card as a practical matter is strictly NVMe, and can be x1, x2 or x4.

There’s no issue with leaving PCIe lanes unconnected. The root complex will sense which ones are and power down the ones that aren’t.

The PCIe lane width negotiation starts from the max number of lanes and works its way down. Lane counts are always a power of 2. So a x4 M.2 card would be lanes 3-2-1-0. x2 would be lanes 1-0. x1 would be lane 0. 

PCIe / NVMe can work over 1, 2 or 4 lanes. So if the host only has 1 lane, you can use that to connect an M.2 card.