Emails limit in the Quarantine zone [Kaspersky Security for Microsoft Office 365]

Is there any capacity limit of mails in the Quarantine zone? If any, can we modify it?

Unfortunately, there is no possibility to customize this setting per user, it is hardcoded in the product (30 days for objects in the backup and 92 days for statistics). 

Is there any limit on the number of emails that can be stored in the Quarantine?

On the KS4O365 side, there isn't a limit to the number of emails that can be saved in the backup. KS4O365 stores only metadata information about the emails in the backup, which is quite small in comparison to the email itself.

Whereas the backup emails themselves are stored in the mailbox (in a hidden folder) on the Exchange online server that hosts mailboxes. When the object is restored from the Quarantine, the email from a hidden folder is simply moved to the inbox.
That being said, the only limit that can be identified in the said scenario is the one from the Exchange online itself (the size of the mailbox for a particular user). I.e. if the total amount of emails in the inbox + emails in the backup will hit the limit of the free space in the Exchange Online mailbox, then the you will need to increase the size of the mailbox or remove the exceeding emails, etc.

The emails from quarantine can be deleted from the Quarantine tab of the console, there you can also sort by date to delete the old ones.