What are the maximum hit points of an NPC?

Question

Every player at a D&D table knows both their character's current and maximum hit points at any given time.

I, as the DM determine the current hp of any NPC that the players encounter, either by rolling dice or taking an average value (as an example, let's say my NPC has 5 hit dice, a size category of medium and a constitution modifier of 1, giving him a starting hp of 5d8+5). But I am wondering: what is the maximum hit point value for any given NPC?

I need this information, so I can determine:

• To what maximum an NPC can be healed by the player's healing spells or heal itself with hit dice during a short rest
• How much damage beyond 0 HP will kill the NPC outright (if I determine it has to make death saving throws, instead of dying at 0 HP)

I see three options:

• NPCs do not have a maximum hit point value
• The maximum hit point value is the same value as the current hit points that you determined, when the NPC appeared (In our example, this would be whatever I get out of the 5d8+5, so for example 26).
• The maximum hit point value is the maximum possible value, that the NPC could have according to its hit dice and constitution modifier (In our example this would be 5 * 8 + 5 = 45).

Are there any rules on how to handle this?

Answer 1

Hit points, as described in the Monster Manual, are maximum hit points

The Monster Manual (page 7) has a section on "Hit Points":

A monster usually dies or is destroyed when it drops to 0 hit points. For more on hit points, see the Player's Handbook.

A monster's hit points are presented both as a die expression and as an average number. For example, a monster with 2d8 hit points has 9 hit points on average (2 × 4½).

A monster's size determines the die used to calculate its hit points, as shown in the Hit Dice by Size table.

A monster's Constitution modifier also affects the number of hit points it has. Its Constitution modifier is multiplied by the number of Hit Dice it possesses, and the result is added to its hit points. For example, if a monster has a Constitution of 12 (+1 modifier) and 2d8 Hit Dice, it has 2d8 + 2 hit points (average 11).

This is not about a creature's current hit points but their maximum hit points. This is how one would naturally read the description, additionally it would be incredibly unusual if this entire section were about current hit points.

Though yes, this is technically ambiguous and could be referring to current hit points, that is not the case. A GM can decide whatever they want for the creature's current hit points, the rules found here are only about its maximum hit points.