Well, it depends. "True" pcie slots do not share the bandwidth, but depending on available CPU lanes, which is awfully few in the last years on Intel - 16 lanes

Well, it depends. "True" pcie slots do not share the bandwidth, but depending on available CPU lanes, which is awfully few in the last years on Intel - 16 lanes in top of the line i7? thats one graphics card - the slots are switched to 8x/8x, 8x/4x/4x and so on when populated. M.2 slots are a whole another story, they are usually connected to the chipset instead of CPU on desktop motherboards (and finding this information for any particular board is difficult). The chipset is connected with CPU through DMI, which equals to pcie 4x and this is shared with everything on chipset - satas, gigabit ethernet, usb 3...