Found a great free tool to help determine the number of end users in a site collection or a farm
According to their website, the SharePointBoost End-User Viewer Tool is a free tool that displays the total number of end-users in a SharePoint Site Collection or Farm and gives the administratorthe ability to delete invalid/inactive user accounts. This tool offers a quick way to check the total number of end-users you have WITHOUT installing it on your SharePoint servers. Simply download the SharePointBoost End-User Viewer Tool and run it by inputting the Site Collection URL, your username in the format domain\username and your password, and the total number of end-users and their account details will be displayed. You can then delete any expired or inactive user accounts, thereby reducing the need to purchase a license for a larger number of end-users.