The rule of thumb standard is the 1-2-3 concrete mix (1 part Cement plus 2 Parts Sand plus 3 Parts Gravel). Works about all the time. Determining how many bags of cement will require knowing the volume of your cement mixer. Most smaller ones used for home DIY projects will only handle about 2 times the above volumes... so 2 volumes of cement, 2 volumes of sand and 3 volumes of gravel.
I usually use a standard garden spade for loading the mixer and 3 spades full of cement is roughly 1 cubic foot (12X12X12 inches) more or less. I find that I can get about .6 cubic feet out of one 80 pound bag of cement.
The easier method (still requires mixing) is to order a product called Quickcrete (here in the U.S.). As the name implies, it is bags of already mixed cement, sand and gravel at the correct mixture for a given project. Simply add water and mix... about 2/3 cubic feet per 80 pound bag of this product... (sorry, we don't do meters).