If you mean the number of units (trackers) you can create in one account: it is possible to create 3 units in one account.
You can see the number of units created/left to create in Dashboard tab - Info - Units.
As to your second question, when creating a unit you should select in its settings the appropriate Device Category and Device Type and specify the ID according to the device you have. After that you will receive Server and Port values that you should configure your tracker to. All these settings you can manage in Unit menu-Settings-Common settings.