The licenses you buy are valid for one year. You should check the monthly reports to view the license usage. You may have to purchase more licenses from PK4 Tech if required.
e.g.: Let's say you purchase the Team edition at $ 1800 in January. In Jan, Feb, Mar you use 50 MAUs each month. In April, May, and June you use 100 MAUs a month. So at the end of June, you would have used 450 MAUs. In July you expect that you will use another 100 MAUs and you expect it to rise further. At that point, you can purchase another 600 user months at $ 1800.
* MAUs: Mobile Active Users.