Doing math to find out how many Shines are intended to be the minimum you need to beat the game, I arrived at 49. There's the one at Delfino Airstrip, Shines 2-7 of Bianco Hills (the developers let you skip the first Shine, though an NPC asks if you're getting ahead of yourself if you go for Shine 2 while in Shine 1, and Shines 1-7 of all the rest of the courses. I know you can use glitches to get the Watermelon Festival early, but if you decide not to do that, I think 49 is the minimum because Episode 7 of every level is all you need to get to unlock Corona Mountain, a name that aged poorly.

So why doesn't anyone do 49 Shine speedruns of the game? Or call them 50 Shine speedruns if the one in Corona Mountain counts?

That category is basically Any% No Major Skips, and that already exists in the category extensions page. (See: https://www.speedrun.com/smsce#Any_No_Major_Skips )

because gelato is a bad stage