How many pixels and resolutions do I need to print a A4 paper picture?

If we were to design a picture for a magazine, how much pixel size and resolution would it take to A4 size?

In general, to the use of coated paper 300dpi is enough, too small print out is not necessarily clear, too big words to consume memory and CPU processing is very slow, so 300dpi is a good dividing line.

The A4 physical size is 21 cm x29.7 cm.

The exact conversion to the pixel also depends on how much resolution is used:

Resolution is 72 pixels per inch, the A4 paper size of the image pixel is 595x842;
Resolution is 96 pixels per inch, the A4 paper size of the image pixel is 794x1123;
Resolution is 120 pixels per inch, the A4 paper size of the image pixel is 1487x2105;
Resolution is 150 pixels per inch, the A4 paper size of the image pixel is 1240x1754;
Resolution is 300 pixels per inch, the A4 paper size of the image pixel is 2480x3508;

other sizes, general standard when printing 300dpi:

A4 paper size of the image pixel is 2480x3508;
A3 paper size of the image pixel is 4960x3508;
B3 paper size of the image pixel is 3248x4300;
B4 paper size of the image pixel is 3248x2150.