On the "Free" plan you get 10 conversions per month. It is automatically refilled each month and you get an email notification once that has been done. If you want to use Filestar more than that you can upgrade your account to the "Personal" plan. It costs 9$ annually and gives you unlimited file conversions with Filestar. You can upgrade by navigating to the "Help" menu and select "Plan".
You can always check your currently selected plan and credit balance in the desktop application if you click on "Help" and then "About" in the menu.