Just the one, I suppose, but any of them will do in a pinch.
The "HD" shotgun is just an 870 Wingmaster that I keep a 20" cylinder bore barrel on when at home. A longer one goes on when out doing sporting clays. I put a side saddle on it a while ago but haven't really run it with that on there yet so I'm not sure how well that's going to work out for me. It'll look goofy on the sporting clays course but that's where I tend to find out what's going to get in my way. I didn't make it to any run-and-gun steel plate shoots this fall, unfortunately.
The only other gun I consider for use in "HD" is a 12 gauge coach gun. It's nothing fancy but I've put a fair number of shells through it doing silly stuff. But, it's fairly compact so it goes with me when traveling in the motor home.