Be realistic and send only the very best.
Track 1 will make or break you so it's got to be superb not just good.
You have about 30 seconds to really impress. If you havn't grabbed the attention of the listener and made him/her WANT to hear more you won't get a booking.
Club and festival organisers get loads of demos and many listen to them in the car going to and from work this is why you get such a short time in which to impress.
Kind organisers try several tracks or listen longer but really busy people give you the magic 30 seconds.