1. Ho many maximum sub-domains can use this SSL, is there any quantity for per-domain?
There
is not limit on how many domains can use a certificate. However, you
can only generate 50 certificates per domain per 7 days according to
Let's Encrypt.
The subdomains from the same domain zone like
example.com and
blog.example.com have the same rate limit. Thus, when the first 50 domains of
sub.example.com get a certificate, the rest need to wait for a week.
2. Is there anyway can use this SSL for mail.DomainName it's also consider as sub-domain.
So
far securing the mail server for multiple domain names (known as SNI)
is a feature that it is not implemented in Plesk because the mail server
does not support it:
Is there SNI support for SMTP/IMAP/POP3? However, you can secure the mail server by following this article:
https://support.plesk.com/hc/en-us/articles/115000179934-How-to-secure-mail-server-with-Let-s-Encrypt-certificate-.
In regards to your other request, it is possible to access emails via
mail.example.com instead of
webmail.example.com. Please refer to the following article:
How to set up an access to webmail over mail.example.com.
