With a WYSIWYG word processor which uses fonts based on point sizes, there are supposed to be exactly 72 points to an inch.
For the purposes of word processing, that is not intended to be 72 points to each inch on the screen, but rather 72 points to each inch on a virtual piece of paper on which the document is being written.
If your virtual piece of paper is supposed to be 8.5x11", with 1" margins, then you have 9" of usable height on the page -- regardless of the size of the screen that you're actually using to "look at" the paper -- and therefore 648 points. With a line spacing of 12 points, that is exactly 54 lines of text.
But it is sometimes tricky to get line spacing of exactly 12 points. "Single spacing" does actually not mean that a 12 point font will have exactly 12 point line spacing, because if that were the case then the bottom extremities of letters on one row of text would be directly touching the upper extremities of the letters on the next row of text -- not very legible.
So, even with single spacing, word processors insert extra space between two consecutive rows of text. Usually, the amount of extra space that is added is determined automatically and is not under the user's direct control. But most word processors also offer an option to bypass automatic line spacing, and instead manually set it to an exact number of points independent from the font's point size.
My suspicion is that you'll want to look for that option somewhere in Pages.