My husband is going to build a pallet coop / run for 2 or 3 chickens (although I may get carried away and expand the flock

It will need to be fairly big as I can't realistically allow them to free range in our garden - I have a veggie patch and a nice flower bed, and there are alot of cats in the area - which means they need a nice big run to flap about in, but not so big that it takes over our very modest garden (it also needs to be relatively tidy looking as we have neighbours either side). I've been keeping an eye out for pallets and have found a local source with more than enough who is currently giving them away. What I'm not sure though is how many we are going to need. I'm thinking probably around 10 to 15 but we've never used pallets for anything before so it is purely a guess. Would you think that that sounds about right for what we want to do?
Any thoughts and tips would be fab, thank you!

Mel