Combien en voulez-vous? = How many/much do you want?
Using the pronoun "en"
The word "en" refers to the item being purchased, so if you asked for bananas, then "en" would refer to how many bananas you want to buy."en" refers to "them". You use it to replace the phrase "de(s) + a thing or things"
Vous voulez des fraises? = Would you like some strawberries?
Oui, je vais en prendre un kilo. = Yes, I will take a kilo of them.
Vous voulez des bananes? = Would you like some bananas?
Non, je n'en veux pas. = No, I don't want any.
Vous voulez des pommes? = Would you like some apples?
Oui, j'en veux. = Yes, I want some.
Combien en voulez-vous? = How many do you want?
J'en veux quatre. = I want four (of them)
Je voudrais... = I would like...
Je vais prendre... = I will take
C'est combien? = How much is it?
Ça fait combien? = How much does it all cost?
Combien coûtent... = How much do/does... cost?