Anyway Plesk has replied:
Developers looked into this issue and confirmed as a bug: PPPM-5050: Subdomains are counted as domains on Server Information page. We appreciate your input to the product improvement and drawing our attention to this.
The fix is considered to be included in one of the next Plesk updates, release notes are available here:
https://docs.plesk.com/release-notes/17.0/change-log/
So far I suggest you:
1. logging in to Plesk database
2. executing the following query:
> select id,name,htype,parentDomainId from domains where parentDomainId=0;
The following article describes the issue:
https://kb.plesk.com/en/129947