EventSentry agents are installed on every server that is being monitored, and there is no limit to how many hosts you can monitor. The only central components are the EventSentry collector, to which all agents connect and submit data. The collector is designed to handle hundreds of connections, so 9 agents is not an issue.
The only other central component is the database, were scalability depends on the database used (e.g. PostgreSQL server vs Microsoft SQL Server), hardware, amount of data stored, database maintenance and expected response time. Again, I wouldn't expect any issues with 9 monitored hosts.
Please feel free to contact support directly at https://www.eventsentry.com/support/request