In this case, you’ll keep all of your unicorns.

Generally, applicable effects all activate simultaneously, and if those effects cause more effects to happen, all those new effects happen simultaneously, and so on. See The Effect Chain for full details.

So when you play Narwhal Torpedo, we look to see what needs to happen. You have 7 pandas in your stable, so the only effect happening is the Torpedo entering. That happens, and all of the downgrades are removed simultaneously. Because they’re gone, there are no more steps in the chain.

If you didn’t have Pandamonium, and you played Narwhal Torpedo as your 6th unicorn, then both it and Tiny Stable would happen at the same time. You’d sacrifice all your downgrades, and a unicorn card.

But let’s say that you played White Elephantcorn instead of the Narwhal Torpedo. Its enter ability lets you give away a downgrade. So you play it and trigger its effect, but nothing else happens in this moment because you have 7 pandas. You give away Pandamonium, which causes Tiny Stable’s effect to be relevant, and thus there’s another link in the effect chain. You have 7 unicorns and Tiny Stable is there, so you have to sacrifice a unicorn. Once that’s done, you still have 6, so you have to do it again.