Dear Zaynab,
If you want to model a single tracker, you should use the "Number of trackers" input on the Array page, and set it to 1 for a single tracker.
The angle limits determine how far the tracker is allowed to rotate. You can use the default values unless you have a reason to change them. For the "tracker elevation angle limits" input, you should use values between 0 and 90 degrees.
Best regards,
Paul.