How do you know if your niche is too small?

On Reddit: https://www.reddit.com/r/Entrepreneur/comments/7myiuz/how_do_you_know_if_your_niche_is_too_small/

On @IndieHackers:
https://www.indiehackers.com/forum/how-do-you-know-if-your-niche-is-too-small-e6eb1ef38b