How to apply discount for bulk order of products

Hello,

I am creating an online store for my client who sells handcrafted glassware (wine glasses etc)

She has 6 glasses, which she wants to sell at ￡110 for a pair, or ￡300 for the whole set of 6.

How can I set up a discount for bulk purchase like this? I assume a discount code upon checkout, but how to make sure the discount code only works when 6 glasses have been chosen?

I want to make sure her customers do not use the discount code when only purchasing one or two glasses!