Field History Limitations
The 'Number of Fields to Track History per Entity' limit cannot be increased manually or without the help of Salesforce Support.
Salesforce's Default Limit: 20 fields per object. (Salesforce)
To resolve this issue, your organization will need to increase your limit by contacting your Salesforce Account Executive (AE.)
See also: Field History Tracking and Storage | Field History Tracking
When Does CaliberMind Use SFDC History Objects?
Customers who purchase funnels often want to refer to the lead history, contact history, account history, and opportunity history to see when certain benchmarks were recorded in Salesforce. CaliberMind allows administrators to set up rules based on these history object benchmarks.
Field History Tracking and Storage
This information pertains to the standard Field History Tracking feature and not the Field Audit Trail add-on feature. Field Audit Trail feature includes provisioning for its required storage and does not count against an organization's data storage usage limits. With Field Audit Trail, you can track up to 60 fields per object. Field history tracking also does not count against your organization's paid data storage limits, and you can track only 20 fields per object. Admins may query StandardObjectNameHistory and Custom object history to retrieve corresponding field history data using Data Loader. See Export field history data with Data Loader for more details.
Resolution
It's possible to use a queryAll() API call or Data Loader's "Export All" option to potentially return history rows beyond the last 18 months however, the data set is not guaranteed to be complete and is subject to deletion at any time.
To track additional fields, you can purchase add-ons by speaking with your SFDC AE – See this article for more information.
See Also: Enable delete of Field History and Field History Archive | Field History Tracking on Record Creation | Increase the 'Number of Fields to Track History per Entity'