Print on Demand

I like writing and don't mind making up picture designs or new ideas. The most important thing seems to find a niche and keep doing it. I kind of had a headache after watching these two videos but I wonder if anyone here has any experience in this field.

Please let me know if you tried this and if you were successful. He makes it look super easy in the video. If it was that easy then everybody would be making 1500 dollars a month selling T-shirts.

Some more specific questions are how many storefronts are you allowed to open in Amazon? Is it better to just use one niche always or to change the niche?

I know I promised to only write meaningful posts here, but these questions are bugging me and I know that Hive people know the answers.