You can refer to this article for capacity planning: https://learn.microsoft.com/en-us/windows-server/identity/ad-fs/design/planning-for-ad-fs-server-capacity
It also has a link to a dynamic spreadsheet: https://adfsdocs.blob.core.windows.net/adfs/ADFSCapacityPlanning.xlsx
If you have Azure AD P1 licenses, you can also deploy Azure AD Connect Health for ADFS: https://learn.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-health-adfs you can do that even if you do not use this ADFS farm for Azure AD/Office 365.
Have a look and let us know!