It would be a little difficult, you could use a global variable to track the number of potions the player has, and then increase/decrease it whenever they get a potion or use a potion. Then when a game is loaded you could check that variable and set the player's number of potions to that amount. If you want to do this for multiple items, it might get a bit tedious.
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Found a little workaround. I'm using right now Yanflys Save Core, and setted it on every map as a parallel event to save the game every 10 frames. This gives the feeling of the game permanently saving, the same goes for using potions and other consumables. Also, I've limited the game to one save slot.
It really starts feeling like Ragnarok Online, maybe you've heard about it.
Perhaps your next plugin would be a "permanent saver" or something similar. Probably with excluding some maps where the permanent saving is disabled. ;)
Greetings
P. s. pretty sure there's another way instead of spamming on every map a parallel event to trigger the hidden saving function, but for now it's working and I'm happy.