###Monsters
What counts as statistics for monsters (at least) is defined in the Monster Manual introduction in the section "Statistics".
It begins by saying:
A monster's statistics, sometimes referred to as its stat block, provide the essential information that you need to run the monster. (MM 6)
and then proceeds to dedicate a section to describing each aspect of the statistics in a short sub-section (MM 6-11).
- Size
- Type
- Tags
- Alignment
- Armor Class
- Hit Points
- Speed
- Ability Scores
- Saving Throws
- Skills
- Vulnerabilities, Resistances, and Immunities
- Senses
- Languages
- Challenge
- Special Traits
- Actions
- Reactions
- Limited Usage
- Equipment
- Legendary Monsters: Legendary Actions
###NPCs
An NPCs stats can be generated in several different ways:
When you give an NPC game statistics, you have three main options: giving the NPC only the few statistics it needs, give the NPC a monster stat block, or give the NPC a class and levels. The latter two options require a bit of explanation.
Using a Monster Stat Block
The Monster Manual contains statistics for many generic NPCs that you can customize as you see fit, and chapter 9 of these rules offers guidelines on adjusting their statistics and creating a new stat block.
Using Classes and Levels
You can create an NPC just as you would a player character, using the rules in the Player’s Handbook. You can even use a character sheet to keep track of the NPC’s vital information.
Class Options. In addition to the class options in the Player’s Handbook, two additional class options are available for evil player characters and NPCs: the Death domain for clerics and the oathbreaker for paladins. Both options are detailed at the end of this chapter.
Equipment. Most NPCs don’t need an exhaustive list of equipment. An enemy meant to be faced in combat requires weapons and armor, plus any treasure the NPC carries (including magic items that might be used against the adventurers).
Challenge Rating. An NPC built for combat needs a challenge rating. Use the rules in chapter 9 to determine the NPC’s challenge rating, just as you would for a monster you designed.
Regardless it is clear that what counts as statistics for an NPC are the same for what counts as monsters and PCs. However, the DM, for simplicity's sake, just has a lot of leeway on how many of those statistics they need to define when making an NPC.