Work item count divides the total number of work items in your sprint between those that have been complete, those that are in progress, and how many remain to be done. By only tracking the number of work items, it disregards any work item estimates and assumes that they’re all weighted the same. When you’re tracking the progress of a project using this, you won’t be able to set estimates on your work items. The Estimate field on work items will be greyed out and uneditable.

Work item count is only used to track progress of work on a board. Learn how to configure time tracking in Jira.