I have HS 85 with 24 inch left hand cutting blades only. Blade guide on top and bottom. Do I need to order 2 or do they come as a set?

I have had a look for you and you would need to buy two, but the one you are looking at is not listed for your machine. Here is the link to the one you would need for your machine: https://www.lsengineers.co.uk/guide-600-mm-24-for-stihl-hs75-hs80-part-no-4226-704-3505.html