If you're having an instance related problem and any of the following are applicable:

• Your world was created and played upon near the steam launch in early 2021
  
• You downloaded a premade world that was potentially made near the steam launch
  
• You've used any mods to mass change terrain (Any time) Every mod that does terrain changes via the console uses the original terrain save state.

you might have a terrain problem. Enabling the console and typing will solve the vast majority of legitimate terrain instance problems. This only does optimization near the player so you'll need to do it at every place edits were done.

If you have instance problems and the above doesn't apply to your issues it will be either

• You've built way too much in a single area. This includes tamed animals and crops.
  
• You've got a lot of dropped loot around that has not been picked up.
  
• You've played around with dev commands and spawned in a bunch of stuff that may or may not be visible like spawner points. Spawning excessive amounts of fish or fish spawners have been known to cause problems for some users while playing with the devcommands.

If none of these issues are related to your issue it's most likely not an instance problem.

If there is no problem and you just wanted a way to reset the terrain to original state: there is a mod for that that uses the hoe and a click while holding left alt. Despite the page not listing it as a requirement, it does require BepInEx, which is mentioned at the bottom of the description page.
https://www.nexusmods.com/valheim/mods/1113

Awesome, thank you all for the helpful info!

Originally posted by Zep Tepi:

The devs updated the game awhile back so that only the end state of terrain adjustments are saved, not every single whack like it used to be.

Source?

Originally posted by umop-apisdn:

Originally posted by Zep Tepi:

The devs updated the game awhile back so that only the end state of terrain adjustments are saved, not every single whack like it used to be.

Source?

https://store.steampowered.com/news/app/892970/view/5411613798265437192

Originally posted by burn:

Originally posted by umop-apisdn:

Source?

https://store.steampowered.com/news/app/892970/view/5411613798265437192

That was changing the order the terrain modifications are applied in; that change has no effect on worlds started since April 2021, and doesn't actually have anything to do with instance counts. It doesn't even say that "end state" terrain adjustments are saved; it only talks about adjusting terrain heights before loading buildings instead of after.

You're talking about something from literally two years ago that isn't at all relevant to the current discussion... and saying it does something that it doesn't, to boot.

Originally posted by umop-apisdn:

You're talking about something from literally two years ago that isn't at all relevant to the current discussion... and saying it does something that it doesn't, to boot.

1) It's plenty relevant to the conversation.
2) YOU don't seem to understand what the terrain optimization patch did.

The new terrain modification system is made to reduce the number of network instances and make loading faster and smoother.

[EDIT] Also, to test:
1) Turn on devcommands
2) Go to an area with a stable amount of instances
3) Optionally "forcedelete 50" to get rid of terrain stuff in the area and "removedrops" to get rid of anything you broke.
4) There is no 4
5) Take out your Hoe, bring up your F2 window
6) Hoe away. The first 3 or 4 times, your instance count will raise. After that, it will not.

This was not the behavior before the patch.

Originally posted by Gontro:

Is there a way to keep the server the way it is and to reset the Instance counters? I assume those are "I used a pick(or hoe) to alter the land and that makes an extra instance", or am I wrong?

Thanks in advance

You are wrong. "Instances" count the objects/creatures currently being processed and/or surfaces/effects being rendered.

The devs updated the game awhile back so that only the end state of terrain adjustments are saved, not every single whack like it used to be.

resetting your instance count would pretty much mean deleteing what you build in range. as all of that is instances.