Pages creates two kinds of documents, depending on the template you start from. One is a typical word processing document. In that kind of document you have little control over page numbering; only being able to choos the style. It begins on the first page, and proceeds from there. It is set up using Document Setup, found under the ... menu.
The second kind is a layout document. In this kind of document everything, even the text, exists in box/area on the page. If you select the paragraph sign (fancy looking P) in the tool keyboard tools and don’t see the Page Break option, you have a layout document. There are more nuanced page number controls for this kind of document.
You probalby have the second kind. To get to the page number controls, swipe from the left edge of the screen to see the page thumbnails. Select one, then open the brush tool at the top right. There will be a Page Number option, and several controls there. Be sure you do not have anything selected on the page iteself, or you won’t get the Page Layout tool.
You can find a better description of this using the Pages Help option under the ... menu, Table of Contents: Format Docuuments > Add Page Numbers.
As you can see from the screenshot below, if you turn off Continue from previous page, you can restart the page numbering from a specified count. This is probably what is happening in your document.