If you’re counting mountains, as above, where do you stop? The sun is 93 million miles away and quite visible. The stars are even further. And why Mt Shasta in particular? And what if you were on Mars and viewing Olympus Mons? There was no explicit mention this was to be from Earth.
At some point, we make reasonable assumptions about a question. In this case, ones that the OP apparently also accepted
Original question: “When you are about 30,000 feet in a plane, How far can you see on a perfectly clear day? For instance, do you think you are seeing up to 200 miles out, 300? Maybe less?”
First reasonable assumption: “Maybe less” means he’s not asking about M31 or anything else out yonder. Forget about that.
Next reasonable assumption: he wants to know how far you can see, on Earth. Unreasonable assumption: he meant to ask how far to the horizon, but he just didn’t express himself right.
Try this: “When you’re standing on top of the Berkeley Hills, elevation 1900 feet, How far can you see on a perfectly clear day?” Distance to the horizon: 50-60 miles. Sacramento is farther than that, so you assure him he can’t see it. And he certainly can’t see the Sierra crest, which is more than twice that far.
If he goes to that viewpoint, he’ll learn better.
You’re not in agreement with anyone in this thread, but keep assuming that you’re reasonable and we’re not. In a plane that is 30,000 feet up in the air, how far away can you see me rolling my eyes? Since we’re in Factual Questions, I can’t really give you the response you deserve, so I’m just going to walk away from this particular thread and let future observers make up their own mind.
If I can give any advice for future you, it’s that when everyone disagrees with you, the odds are pretty decent that the mistake is not on their part.
Good plan.
If the OP had been asking how far one could see from 30,000 feet to a particular mountain, they would have presumably named the mountain. If they were asking what’s the farthest one could see from anywhere on earth at 30,000 feet to any mountain, they would presumably have expressed the question in something like that form, mentioning the word “mountain” or even “Everest”, since the relevant answer would involve looking at Mt. Everest. (I don’t see how Mt. Shasta could possibly be relevant.) The lack of any such wording makes it pretty clear to me that they were asking about the distance one could see at any arbitrary point on earth, disregarding local topological features that would make the answer different at different locations. So yes, the question was how far one could see to the horizon.
This thread was started 19 years ago, it’s old enough to vote. We are the future observers.
And I’m pretty sure we already made up our minds. 
ETA: Ah, shit. I was supposed to walk away.