In my MPLAB project, when I debug I see xNumOfOverflows increases about once a minute.
Could someone explain the meaning of this counter and if this increase is normal?

Sounds like you are using a 1 ms tick rate and a 16 bit tick counter. xNumOfOverflows is just a counter of how many times the tick counter has wrapped, and isn’t a problem. Within FreeRTOS time is measured in ticks, and you can specify durations up to the maximum storeable in the tick counter. It handles everything that needs to be done when the counter wraps. Sometimes programs may want to log when events happened to a longer scale than the tick counter, in this case you can use a combination of tick counter and the overflow counter to remember when something happened. But unless you need something like that, you can ignore the overflow counter.