How do Scav spawns work?

Not all bot Scavs spawn at the start of the raid (some are always present), they'll slowly spawn in through the first ten minutes of the raid into a set of predetermined locations. This is done to prevent spawnkilling while your PMC loads into the game. Customs, for example, may not have the full set of scavs spawn near Scav Checkpoint as your character may also spawn there. Check back in five minutes they'll likely all be loaded.

More scavs do spawn in a raid, but they aren't bot scavs. They're player scavs! Players can spawn late into a raid at a different set of predetermined spawn points. It doesn't seem to be a fixed number of guaranteed spawns, rather it roughly correlates with the population of the Tarkov server you're playing on. Different maps will have different caps on the number of player scavs that can spawn. Likewise, these player-controlled scavs spawn at different times ranging from about five minutes from the start of the raid to as late as ten minutes before the end of the raid.

Scavs do not spawn based on the player's location, although some may spawn due to the player's actions or other external factors. On certain maps, such as Labs, heavily-armed Raiders will spawn when you call for any extract. On Reserve, a train automatically arrives which serves to drop off more raiders while also becoming an extract location itself.

Sniper scavs do behave differently than regular scavs. They cannot move down from their exposed position. These scavs can stand up or crouch, but will stay on their rooftop while they try to shoot you.