A track is like a trail of breadcrumbs; it records where you have been.

The unit stores 10,000 trackpoints internally. By default, only 3,000 are shown but that can be changed in the Map setup/Tracks tab.

Essentially unlimited storage can be used by storing tracks on the memory card in setup.

The Garmin eTrex Legend Cx keeps three kinds of tracks: active, saved, and stored.

Active tracks have elevation and timestamps. They are the most 10,000 most recently recorded position points. Active tracks will either overwrite the oldest points if set to wrap or will stop recording and display a message if you set it to fill.

Saved tracks are created manually by the user from the keypad. Saved tracks are 'compressed' by the unit to have under 500 trackpoints and have neither elevation nor timestamps and have fewer stored trackpoints.

Optionally, the unit may write tracks to the removable SD memory card as GPX files. These files are immune from the limit of 10,000 points, the settings mentioned above and the compression of saved tracks so they always have timestamps and alttudes. These files can be read by your computer, but the GPS itself will never display them.

Because some of the tracks are saved tracks.

Menu>Menu>Tracks>Setup>Data Card setup> Check the box for "Log Track to Data Card". Now the unit will create a GPX file on your memory card each day containing all the saved data (including elevation and time) for your activities. Those files can then ba accessed via the USB mass storage mode or via an external card reader.

The exact number is hard to pin, but in short, "lots".

Your memory card is also used for maps and custom POIs. As it's a FAT-based filesystem, there is also some per-file overhead and rounding to nearest blocks and such. There's some overhead in the GPX file and there's some overhead per track and per track segment. As the data is stored as readable text, each individual trackpoint may even vary in size - for example elevations in the mountains have more digits and thus consume more space than elevations near sea level. Longitudes neare the prime meridian have fewer digits than those near the date line

Now, with that weaseling out of the way, we can still glean some numbers. In a survey of 147 .gpx files, there were 167,674 trackpoints. The number of bytes in each file total 19,685,455, so we get approximately 117 bytes per trackpoint. That comes to almost 9,000 trackpoints per megabyte. So if you have the unit set to record trackpoints at the maximum rate of 1 per second and are logging around the clock, budget about 10MB of track data per day.

New files will be created at midnight GMT. One file will be created per day and individual tracks will split within that file in the standard GPX way of wrapping them in trk/trkseg tags.

That's a lovely question for Garmin. The reality is that the saved tracks - whether internal or as GPX on a memory card - contains none of that info.