Everything
@21hens-incharge said x2.
@HeatherKellyB is right, chicken math is real. Plan for more, then as 21 said, ventilate for a few more than that. Its easier if you use broad, covered eave vents. Literally open eaves with hardware cloth affixed underneath via screws and washers and a ridge vent - 2x4 rafters on edge provide .3 sq ft of breathable space per linear foot of length. Broad overhangs also shelter windows from blown rain, and provide additional shade for cooling. Additionally, eave vents - unless you build a monitor roof with very high roosting bars - provide no danger of drafts on birds in sub freezing weather.
Chicken wire keeps birds in, it keeps nothing out. As
@David61 says, hardware cloth (secured via screws and washers).
If you are making a raised hen house, consider the height of your wheelbarrow when setting the floor, and how you will open one wall - to make cleaning easier, since you can't walk in. If you are setting the house on the ground, drainage, drainage, drainage, DRAINAGE!
What are your avian predators? Do you need a covered run or no? What are your ground based predators? Will hardware cloth do the deed, or do you need hardware cloth and something more substantial for the run? Did I mention DRAINAGE for the run?? What's the climate? Should you have a way to hang a tarp/sun sail/etc off one side to provide additional shade?
Roofing materials... What's the budget? How are your skills and tools? How often do you want to maintain it? What loads (SNOW???) do they need to support?
and finally, building in multiples of common dimensions helps reduce cuts, which makes building go faster, and reduces waste.