Getting lost in the heft of Stephen King's 'It' is half the fun — that thing is a proper doorstop. The original hardback first edition published in 1986 runs 1,138 pages, which is what most collectors and bibliographies list for the Viking/Putnam first printing. I still laugh thinking about lugging that copy on a weekend trip and realizing I was carrying a small novelistic planet.
That said, page counts vary across editions: paperback reprints, large-print versions, and overseas editions change the total because of typesetting, font size, and paper. If you're hunting for a true first edition, check the publication page and the 1986 imprint; the 1,138 figure is the one people usually quote when they mean the original edition of 'It'.