What does "hot number" mean?

Also, hot stuff. Someone or something that is currently popular or fashionable; also, someone or something unconventional or daring. For example, That new song is going to be a hot number, or He really thinks he's hot stuff. These slangy expressions use hot in the sense of “recent” or “fresh,” a usage dating from the 1300s, although hot stuff dates from the 1700s and hot number from about 1900.