A purchase request list report includes columns for case ID and regional cost center.

To show the total number of purchase requests for each of the regional cost centers, you need to summarize the case ID column by count and group the results by the regional cost center column. This will create a summary report that displays the count of case IDs for each cost center value. References: https://community.pega.com/knowledgebase/articles/reporting/86/summarizing-data-reports