One Sheet, Two Pages
A single sheet of paper has two sides. Each side is considered one page. So a sheet of paper is two pages.
One Sheet Folded, Four Pages
A sheet folded in two is a four page document. Page one is the front cover, page two is the inside front, page three is the facing page, and page four is the back cover.
Two Folded Sheets, Eight Pages
Two folded sheets that are nested together are an eight page document.