Kingdom Hearts III is different from previous entries in the series in terms of the amount of Save Slots you’re given to use.
Previous Kingdom Hearts games had up to 99 Save Slots that you could use to save your game, however Kingdom Hearts III has just 9 Save Slots, a huge downgrade.
We don’t know the exact reason for this but many assume this to be because of the in-game camera, which you can use to take selfies and other pictures.
Even the previous game based on the same engine as KH3, Kingdom Hearts 0.2, had 99 Save Slots. However that game didn’t have an in-game camera like this one, so maybe that theory makes sense.
Not to be sad fellow fans as this is the modern era and I’m sure the developers can patch in more save slots if there’s demand and if they want to.