some laptop have 2 slot for M.2 (max at most) no limits as u mention it might be for storage - subjected to max capacity available for each m.2 (2tb so far biggest).
M.2 can be sata or NVMe interface. and for Nvme it comes with 3 different length 2240, 2260 and 2280...  Lenovo for example, they use 2242 + 2280 (extra slot - depends with model and product range)