entity-tracking-range

The entity tracking range basically tells the server how close a player has to be to an entity (mob, other player, etc) before the player can see the entity.

You can change it in spigot.yml, under world-settings -> default. Its default value is:

entity-tracking-range:
  players: 128
  animals: 96
  monsters: 96
  misc: 96
  display: 128
  other: 64

Recommended starting value

The default settings are good enough for most servers, so you can start with these:

entity-tracking-range:
  players: 128
  animals: 96
  monsters: 96
  misc: 96
  display: 128
  other: 64

If you’re running a larger server and want to reduce the number of entities being sent to a player, you can lower the values to:

entity-tracking-range:
  players: 128
  animals: 40
  monsters: 80
  misc: 48
  display: 48
  other: 32

I’d recommend always keeping players at 128, as lowering it can cause look a bit strange.

What does entity-tracking-range do?

As mentioned above, any entity which is outside of the tracking range (in blocks) from a player won’t be sent to the player. The player simply won’t see the entity.