Depends where you lived. They'd use the local track's total handle for the day, or in some instances the tote, or in other instances the totals for three races at a time (with a 9 race card that would give you three digits for the day, so you could theoretically play just one number at a time).

Pre-1985, numbers was a HUGE moneymaker. Then the States started with the legal numbers and that was all she wrote. There are still some dinosaurs who book the number, but years ago there were numbers stores and sheet writers all over the place. Everyone in the neighborhood knew where to get down. Today people who play, and book, the street number, are few and far between.