The PowerPoint presentation in this link describes how to use EndNote Term Lists and the Subject Bibliography to conduct text frequency analysis of fields within EndNote.
The way to achieve subject bibliographies in EndNote X20 is described here:
Select Tools
Subject bibliography
Select keywords and tick the bottom box ("In other fields")
Select OK
Displays terms and number of records
Select all, OK
Then need to format the results - to just get the terms and frequency
Output style: annotated
Layout, Terms
Subject terms only
Inlcude subject term counts
Subject term layout - by term count descending
This produces a listing of the indexing terms with frequency in brackets which can be printed or saved