How many pixels does it take to take a 4″ x 6″ photo with a camera? How large of a photo can you typically print for a picture on a web page? (536 x 750 pixels)

To achieve the ideal print resolution, the pixels of the photo should reach the accuracy of 300dpi per inch, as you said, to output a 6-inch photo (4X6) then the pixels of the photo should reach the resolution of 1800X1200, lower than this value, the quality of the photo will be reduced, and higher than this value, there is no improvement in the quality of the image (because in the civilian field, even if it is professional). Output devices, that is, to achieve the accuracy of 300dpi, and then higher only in the military, medical and other fields, inkjet printers advertised 2400dpi/4800dpi ........ (and this is not a concept, do not confuse).

You said "536X750" size of the photo, is a nearly square rectangular photo, the normal output is a high 2.5 inches, 1.7 inches wide photo, if you want to zoom in on it, the image will become "false", if the image quality, precision requirements are not too high, zoom in to 5 inches. If the image quality and accuracy requirements are not too high, enlargement to 5 inches output is still acceptable, 6 inches to be worse. Of course, this also depends on the image quality of your picture, if the picture itself is a little "false", that 5 inches may not put out.

And, that is, if you are required to view from a distance, you can not care about these, can be done at 72dpi per inch, out of the kind of precision as if outdoor spray painting, far from good, close to a blur. The output size is about 12 inches or so.