[DiapDealer]

You're talking about a function of the reading app. You don't need to create "separate pages" in Sigil (or most ePubs). That's what reflowable text is all about. Depending on font, font-size, margins, etc..., a "page" will be different on all devices/apps. But you don't have to do anything to make it work. The "Next" & "Previous" buttons will automatically present your text to the reader in "pages" on almost every single piece of hardware out there.

Don't rely on calibre's viewer as an indicator of what your ebook will look like (or how it will function) on most devices. Use the ADE desktop software to preview if you don't have an ePub device.

If you're wanting to make a "fixed-layout" ePub (where you decide exactly what's on each and every "page"), that mirrors the pagination of a print book (or PDF), then that's a different story.