Nikolai,
There are several considerations. First decide on the number of copies in that edition. You may feel that the particular image at tht specific small size will be printed as an open edition. Larger prints, perhaps that you have spent more effort on or are very unique, you might limit to 14 prints only. That gives a sense of uniqueness and an idea that perhaps the print will increase in value.
So the numbers have a coded meaning.
One can wite the number of the print in a sharp #2 pencil in the bottom of the picture. i've seen both left and right; right I prefer.
If the print has no border, then do it one the back and also mat the print and do the same on the innner lower edge of the mat near a corner.
Asher