It's simple no one can answer the question asked because they don't know enough details to work it out.
6 cameras mean nothing.
Without knowing resolution frame rates and compression, with compression it varies significantly with the type of compression and how busy the scene the camera is capturing.
But if you base it on the 4MP using up 40GB a day no idea of frame rate or compression used or how busy the scenes are.
6 cameras of the same would use 240gb a day, so 1TB would be 4 days. And 2TB would be 8 days as a rough guide.
To that end it's probably easiest to fit the largest single HDD you can afford that the NVR or DVR will support. Some units will support multipul HDD.