A "Sheet" in Numbers does not have a fixed dimension. It is as long and as wide as needed to contain the Table(s) and other objects placed on it.
If the 'occupied' width or 'occupied height' of the Numbers Sheet exceeds the physical width or height of the print area inside the margins set for the paper it is being printed on, the print software will automatically split a table to place as much of it as will fit on the first sheet of paper, and generate one, two, or three new pages to contain the parts of the document Sheet that will not fit on the paper page.
To avoid this, you need to review and possibly adjust the margin settings for the 'page' as printed, and/or move the objects on the Sheet into a more compact group, and/or rescale the contents of the Sheet to fit the paper, using the tools Wayne mentions in his post above.
Regards,
Barry