From the JSON readme:
tracks: statistics on aircraft tracks. Each track represents a unique aircraft and persists for up to 5 minutes after the last message from the aircraft is heard. If messages from the same aircraft are subsequently heard after the 5 minute period, this will be counted as a new track.
Look at it as the number of aircraft coming into range per hour, whether that is newly arriving in your receiver range or coming out from behind an obstruction after a gap of at least 5 minutes.
It’s not necessarily a useful metric for assessing receiver performance since the figure changes so much with aircraft behaviour - a steady stream of aircraft arriving and landing that you can see continuously might show a similar number of tracks to only a few aircraft flying around and going in and out of coverage.
It’s a figure that is going to be fairly unique to your location and receiver set up, since things like obstructions will affect it quite a bit.