Where and how do I keep the extra buttermilk biscuits?
Biscuits are notorious for that; they're best eaten right away. If you want to preserve their original texture longer than 12 hours (yes, that short of a time), your best bet is to freeze them as soon as they are cool and eat them within a month or two. Wrap them as air-tightly as possible. 

You can pop the frozen biscuits in a moderate oven or unwrap and give them an hour or so on the counter to defrost, then warm them. Either way, it's best to serve them warm, they will seem softer and fresher that way.

If you've found a recipe you like and think you might want to make them frequently, mix the dry ingredients for multiple batches. If you can weigh the dry mix that you have just mixed, then you're golden. You can make as many or as few biscuits as you want anytime just by using the same proportion of dry, fat and liquid as the original recipe. You can easily make a single biscuit that way or feed a crowd.