Thread length

How many posts does it take for a thread to get another page?

AFAIK The thread starts a new page on the 50th post.

Close. The fiftieth reply starts a new page. Thus there are fifty posts per page. The OP and 49 replies on the first, fifty replies on each additional page.

So the respective sizes of the posts are completely irrelevant?

Correct.
As long as we’re on the subject, what is the record for the longest thread (most replies)? There is a semi-active thread in MPSIMS (Jonathan Chance’s Dope-a-Ween thread) that spilled onto 11 pages, with something like 505 replies.

Actually, “semi-active” may not be the best characterization of the thread. Semi-active makes it sound like it 67 years old and is looking for a nice quiet condo in a community with a center that offers ceramics classes on Monday nights and water aerobics three times a week.

No, this thread is sort of like a neutron star collapsing under its own weight.

Wait, it’s more like a Marlon Brando kind of thread. It just kept growing and growing, engorging itself until it became this lump, unable to get out of its own way. And besides, with the dope-a-ween over for almost a week, the thread, like Marlon Brando, is way out of date and past its prime.

It currently languishes on page 2, behind the more popular threads-again, like Marlon Brando.

After 10 pages, vB starts getting a bit wonky, so the mods usually close them at that point, and either start a new one themselves, or ask that the OP do so. For a current thread, then, to be fair, you’d count all the sequels as part of the same “meta-thread”. I suspect that the record holder there would be “Ask the Gay Guy”.

UBB had no such length restrictions, so there were some threads considerably longer, but they were mostly before my time.

Guy Stuff and Flat Earth.

One of 'ems gotta be the longest.