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>
There is no exact number. These are the relevant figures:

2000 - recommended maximum, after which there may be a performance decrease (Both [On-Premise](https://technet.microsoft.com/en-GB/library/cc262787.aspx?f=255&MSPPError=-2147217396#SiteCollection) and [SPOnline](https://support.office.com/en-gb/article/SharePoint-Online-software-boundaries-and-limits-8f34ff47-b749-408b-abc0-b605e1f6d498)). This figure in reality depends on hardware/subscription level.

250000 - hard limit for [On-Premise 2013](https://technet.microsoft.com/en-GB/library/cc262787.aspx?f=255&MSPPError=-2147217396#SiteCollection) (There does not appear to be a published hard limit for SP Online)

100000 - hard limit for SharePoint Online as quoted by [Metalogix](http://www.metalogix.com/Blog/Blog-Article/metalogix-software-blog/2014/09/17/sharepoint-online-ups-site-collection-sub-sites-storage-and-file-sizes-beyond-sharepoint-on-premises), but there is no other evidence.