Board Thread:Deep-Sea Prisoner Discussions/@comment-31943407-20170722005102/@comment-25955276-20170722033620

Hard to answer the question, since no one here understands Japanese, at least enough that they might frequent Japanese forums or other sites where Japanese fans of DSP frequent.

https://comic.pixiv.net/works/1749

The closest we can number it is based on the followings for the first Wadanohara manga - assuming that at least a majority of the followers of the Wadanohara manga are Japanese and found the manga due to playing the game beforehand/because of DSP, we can probably say there's a couple thousand fans at the very least.

There's also the merchandise, as you've said. It's been printed and a number have bought it... so we can probably assume it's popular enough that people actually wanna buy it.

I'm relatively unaware of people who create free RPGs, but I believe DSP is quite popular considering they've managed to make some profit/attempt to make some profit off of their creations.