Zoho MarketingHub Leads limit

Zoho MarketingHub Leads limit

Hi, I would like to know how to increase the limit of Leads we can have in Zoho MarketingHub.
In the section settings->general->subscription. It appears "Zoho One Enterprise (5500 Leads)".
Using Zoho MarketingHub we reached a number of leads over 5500 and then we saw a pop-up asking us to pay more. Consequently we deleted our old Leads for having space for more.
How could we increase that limit and how much does it cost?