I don't have an unslabbed #1 to flip though, but a silver-age 12-cent Marvel of that era has 16 double sided pages, not counting the cover. To count for completeness, you should count 8 pages from the splash page to the centerfold (where you see the staples). You could continue and count 8 pages from the centerfold to the back page but it's not really necessary.
There are 36 pages to X-Men 1 which include stories and ads. I just counted; I have an unslabbed copy. There are 23 pages of the story and thirteen pages of ads including a full-page in-house ad for Spidey #4.
Very interesting. I've just counted the pages on about a dozen 60s comics and they are all 32 pages. So, I guess they decided to put a little more in X-Men #1. I wonder if there are any other books where they added an extra 4 pages.