We recommend you start with the free 14-Day Trial edition since it will allow you to test all Premium features for up to 14 days. The Trial and the Basic edition are limited to merging and validating up to 10 workbooks, where the Premium edition has no limit.
You can compare all editions at https://wbmtools.com/wbmerge/Documentation/WBmerge%20Edition%20Comparison.pdf
Once your free 14-Day Trial has expired, you can decide if the free Basic edition will work for you or you may elect to purchase the Premium edition. You can always move up to a Premium edition anytime in the future.