What does combien de stylos as-tu mean?

Combien de stylos as-tu means 'how many pens do you have'.

Note. strictly speaking un stylo is a ball-point pen; however fountain pens are so rare these days that stylo is used to distinguish a pen from a pencil. A pencil is still called 'un crayon'.

ps The french have a perfect right to call a ball-point what they wish ; after all they invented it.