Question: I'm trying to figure out continuous "Unable to write SPDistributedCache call usage entry." error in ULS and some instructions like [this](http://www.sharepointfire.com/2013/11/the-specified-user-or-domain-group-was-not-found/) suggests that issue might be with super accounts having incorrect username.

The user accounts used for superreader and superuser accounts are currently set in domain\superuser and domain\superreader format to web app's portalsuperreaderaccount and portalsuperuseraccount properties.

But I'm wondering if they would need to be updated into i:0#.w|domain\username format instead to fix issues?

As far as I know these both formats should resolve fine but thinking if this still could be an issue.

Answer: 
I think 2000 subsite per site collection is just a threshold value.

There will be impact on performance if your subsite count is more than 2000.

Thats why Microsoft recommends upto 2000 subsites per site collection.

<https://support.office.com/en-us/article/SharePoint-Online-software-boundaries-and-limits-8f34ff47-b749-408b-abc0-b605e1f6d498>
I think that the official source is <https://support.office.com/en-us/article/sharepoint-online-limits-8f34ff47-b749-408b-abc0-b605e1f6d498>
and the last updates (August 2018) confirming that the Subsites limit for a single SPO Site Collection is 2000.