Licenses you buy is valid for one year. You should check the monthly reports to view the licenses usages. You may have to purchase more licenses if required.
E.g: Let's say you purchase 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.