The DPI settings and requirements have nothing to do with the numbers of inks. You can have 4, 6, 7, 8, 12 or whatever, but this does not change the printer's resolution, which is only the total numbers of dots the printers can lay down in a square inch. Epson uses 1440/6 as a basis for printing at 1440 DPI. Similarly, if you print at 720, use 360 or 180. All these numbers mean that the printer's driver does not have to go through odd numbers of interpolations to achieve the desired print quality. While 300DPI is ideal for offset printing, it is not necessarily optimum for inket printing.