What is the meaning of take a number?

it means to wait in line - from the practice of businesses giving people a number when they arrive and then calling out the numbers in order.

This phrase is often used sarcastically when someone wants to complain about something, and the other person has already heard enough complaining, or if they want something and he's got a lot of other people wanting things too, so he says "Take a number" or "Take a number and get in line."