How many different types of bullets does it take to kill a deer?
It don't, but I'd not be expecting RN 338's designed for said cartridge to be around for long.
Then again, can't come up with a reason in the world that one would need a 338 for deer.
Even if you are right and everyone quit making bullets tomorrow, the internet has changed the situation with availability of rare and hard to find items. I wouldn't be too scared to shoot "rare" bullets or cartridges today. With a little diligence, you can find them.
A bigger concern might be the change in laws making it illegal to shoot bullets containing lead in the hunting woods. Then, yeah, you've got a problem.