Newbie looking for a manual or tutorial on prompts for image generation

Hello, I am just getting started with writing prompts for image generation. I'm using Kalon.ai in free mode (about 5 images per day per account) but I'm happy to try other AIs. My problem is I haven't found a basic guide or tutorial that will answer questions like: - Why does the same prompt sometimes produce completely different results, e.g. I specify two people in the image, but often I get 1 or 3? Or I specify "clothing" but I get no clothes.