How many endings are in Slay the Spire?

Solution 1:

There are 3 possible ways a Slay the Spire run can end.
In your sense, counting each character individually, there would be 6 possible 'endings'.

Note: Details about this might spoil the ending for new players, so the answer is hidden behind spoiler tags.

1:

You die before beating the final boss (or bosses if you play on A20).
This counts as a loss, so you cannot unlock higher ascension levels.

2:

You beat the Act 3 boss, but don't have the keys to continue.
When this happens, the game ends, but it gives you a hint that there might be more to the spire.
Note that this does count as "beating the game", you can unlock a higher ascension level.
After doing this with each of the first 3 characters, Ironclad (red), Silent (green), and Defect (blue), you unlock the ability to get their 'key'.
Collect all three keys not to be zapped by the heart in the final screen, and instead continue to the secret 'Act 4'.

3: (4/5/6)

You beat the Heart, the final boss.
When you do so, a little cutscene comic will display, these comics are unique to each class, so you could consider this a different ending for each. Then the game's credits will roll. Neow congratulates you, and you can play again if you so desire.
This ending can unlock higher ascension levels just like the previous one.