You may have imported a track or created one and this has over the 2,500 maximum you are allowed in a Garmin GPS unit.
If this is the case you can 'filter' them to the number you are wanting. To do this -
NOTE: When you filter track points, the removed points are permanently deleted from the track.
1. Right click on the track (when it is in the bottom left pane in Garmin BaseCamp)
2. Select - Filter selected track
3. Select an option: Select a point. ...
4. Select 'Entire track' under Filter.
5. Maximum points - change to 2,500
6. Click OK