ICloud – Will the calendar and contacts in iCloud be lost if I delete backups

• Start Contacts on your Mac and export/archive all contacts (as a backup)
• Go to System Preferences -> iCloud and disable syncing of contacts (and anything else you don't want to sync to iCloud)
• Log in to iCloud.com directly and delete any contacts stored there

The data repository for Calendar service is located at /Library/Server. Inside that directory is a folder containing data specific to the Calendar service at /Library/Server/Calendar and Contacts. Within that folder is a Config folder that has the main configuration file for the caldav daemon called caldavd-system.plist.

The limits are based on your configuration and physical capacity. Outside of that users can have as big a Calendar and Contacts as they want/you want them to have. Apples limits are arbitrary for use of their service.

EDIT There is a difference between limits and compatibilities.

BONUS If you want to learn how to set up your own limits try here.