Why do people say they are going to sing a "number" or dance a "n

Question:

Why do people say they are going to sing a "number" or dance a "number"?

Best Answer - Chosen by Asker: During the development of performing arts, in the days of the big bands and even small combos, the songs they were going to play had names and they also had numbers. The band leader would simply call out a number to the orchestra and it was easier than using thre title. it's much the same idea as using codes, numbers, for law enforcement communication. in the early days of musical orchestrated performances, a "number" was literally a song identified by its NUMBER.