Means, a lot of people are making demands on my time right now and your request may or may not get granted depending on if and/or when I get a chance to get to it.
"You want that when? Yeah, you and everybody else. Take a numberJack."
Figurative: "If you want to discuss this topic with me, you will need to wait your turn, as there are any number of others who wish to do so who got here before you did."
Comes from a line control mechanism that is found today with decreasing frequency, where one is requested to take a numbered ticket from a strategically placed dispenser and accordingly wait for that number to be called.
Your computer just blew up? Take a number, I've been getting calls on that all day.