Don't want to write this by hand? GodCraft sets minecraft:follow_range for you with a switch.Get GodCraftOverview
minecraft:follow_range is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Sets how far away, in blocks, the entity can notice and track a target it wants to follow or attack.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:follow_range": {}
}