In my Alfheim campaign, I make it relatively common for a player character who braves the dungeon on several adventures to get to two hearts (20 HP) but extremely difficult for PCs to get more hit points. Heart stones are frequently the first milestone reward my players see, and at my table they do not “stack”; every PC needs one, but one is all they need. This matches my humanoid (“non-monster”) bad guys and other significant NPCs, who frequently have two hearts of HP but then rely on healing, armor, and other buffs if they need to be tougher mechanically.
They way I see it, RPGs are a balance of drama and agency within a story narrative. There have to be stakes, and the decisions that players make for their characters need an opportunity to matter in game.
Early in the campaign, when PCs have only one heart and little to nothing in the way of buffs or bonuses, they know and accept that their characters are squishy and that sometimes the dice will do them a dirty deed. Two good weapon strikes or a single critical hit from a lucky foe will take a PC down with little to no agency if they put that character in harms way—which is what they are supposed to do—even if they don’t make a “tactical error” or other blunder. That situation is definitely dramatic but can offer little or no player agency because things happen so fast. The players’ only solace when bad things happen is that they haven’t invested too much in these still-new characters. They will get chances to roll death saves, which is certainly dramatic, but often when PCs go down, the real agency shifts to other players’ characters making daring “don’t die on me, man!” runs to stabilize them.
For the player who has made the good calls, gotten through a few adventures, and kept his/her squishy one-heart character alive through sound tactics (and some good luck), I think a heart stone is a just reward. With such a piece of loot, the player has now opened a space in the game to fight bigger, nastier, more plentiful opponents while widening the dramatic zone of uncertainty that makes turn-by-turn decisions in combat really count and makes the changing narrative of a fight more exciting. Two and a half percent of all successful melee weapon hits (1 in 40)—without benefit of magic, abilities, or effort bonuses—will straight-up drop a one-heart character (D6+D12 averages 10 HP). As an alternative, instead of going from hero to zero with a single devastating hit, a fully rested PC is more likely to get badly bloodied with the bad guy lands a big hit, and it’s decision time on the next turn: retreat, risk losing time to pull back to heal (with or without help) and return to the fight, or tank the damage and hope to drop the bad guy first; these are generally exciting choices that maintain player engagement and agency while still keeping the stakes in combat encounters.
Especially when facing powerful magicians or monster foes who can deal out even more damage, the eventual acquisition of a few heart stones for PCs with 20 HP seems to have worked pretty well at my table so far.