Strictly IMO you can replace one hinge - you're going to have to wrangle with the hood for a while anyway, due to the multi-positional adjustment of the hinge position in relation to the fender, front panel, and cowl grill gaps. I replaced both of my '69 Pacer hinges at the same time, as both of them were loose and bent to boot, and it took a good 3 hours by myself to get the hood back into the same fit as before - I would suggest you get a good friend who doesn't mind hearing a few mild swearing episodes to help you when you do replace one (or both). Use white Lubriplate to grease them - WD washes out the lube and dries off, leaving you worse than before. The grease I used (matched the factory lube on them when I got them) has been on there better than 10 years, still looks and works as before. Leave your hood up when the car is stored - sounds weird, but watch the springs when you close the hood - they will stay stretched if you close it, eventually weakening them.
I was fortunate to be able to get new GM hinges before the price trip to the moon - I'm really thinking about getting my old hinges rebuilt for spares (I don't trust repops). Good luck with the hunt for new ones -