You need not buy additional Salesforce user licenses for all dftly Time Tracker users. Mobile/web Time Tracker users do not need a Salesforce license.
Mobile Users only login Time Tracker using their Phones and Web Users can login Time Tracker on the PC.
The people that need the Salesforce licenses are the Admins and the Managers who need to track the teams that are using the Time Tracker mobile app. If you want your users to be able to track their time within Salesforce, then they too need to have Salesforce user licenses.