How to calculate the number of Plesk instances required for a huge amount of mail accounts?
There are no limitations regarding mail accounts in any Plesk edition. However, the allowed amount of domains is different for all 3 editions Plesk Web Admin Edition, Plesk Web Pro Edition, and Plesk Web Host Edition.
If it is planned to create all the mail accounts for the number of domains from 1 to 10, then one Plesk installation with any edition would be enough. Otherwise, calculate the required number of servers according to the domains limitations of a particular Plesk edition.
Refer to the following article for more details regarding available Plesk editions: Plesk available editions and differences between them.
The information about hardware requirements is available here: Hardware Requirements for Plesk Onyx.