A working rule of thumb is one image for every thousand words. Applied to a planned series of 45,000-word primers, that gives each book about 45 pictures. This is a personal publishing judgement, not a universal rule. The aim is to include enough illustration to make the book appealing while keeping the text central.
Notable Point · 1:24-1:51
A practical ratio of one picture per thousand words
A working rule of thumb is roughly one picture per thousand words, which would give a 45,000-word primer about 45 images.