Number of render slots In this old video Blender has 10 render slots: https://www.youtube.com/watch?v=bfMJ9NDpXAI

However, in my new 2.78 version, I see only 8 render slots. Is there a way to set the total number of slots? Actually, I would like to have more than 10.
During this commit a constant named IMA_MAX_RENDER_SLOT was introduced and set to 8.

You'd have to change the line

#define IMA_MAX_RENDER_SLOT     8


in source/blender/makesdna/DNA_image_types.h to something else and re-compile to change the number of slots.