By statute it has to be in the original box, but you can fit more without boxes. I like Sellier & Bellot boxes, but not Blazer. The Cabela's ammo cans in the photo are the older weaker type. I bought a few of them for $7.50 They are not really sturdy or stackable. The newer version is better and I almost bought a dozen when they were on sale for $7.50, I waited too long and now they are again $15. However, I like MTM much better anyway. So for that kind of money I’d go with MTM AC11. Surplus 50 cal are good too. You can put them closer together, but they are much heavier. Keep in mind that when these are loaded they weigh a ton.
RE: OP
You can keep 10K rim fire + 10K center fire + 5K shotgun in one building. Above that you’ll need a permit. See 527 CMR 13.04
http://www.worcesterma.gov/uploads/J9/aM/J9aM8tDhs-AAMC_h6CGk3g/527013.pdf
How much you keep is in part driven by how much you shoot and your budget. If you shoot few boxes each year than keeping 1K might not be advisable. As for the shelf life – if you keep it in water/air free environment, it will probably outlive your grandchildren. Few years back I shut some WWI 7.62x54R and I have shut a lot of Korean War 30-06. Unless you keep it in a swimming pool, 20-30y rotation will ensure you have fresh ammo.