Can I add a Numbers spreadsheet to a Pages document?

There is no embedding of the Numbers Spreadsheet, but you can enter a table in Pages. In Numbers, copy and paste the selection range from the Numbers table, and back in Pages, single-click the first cell of that table, and then choose Edit > Paste. Pages will autoexpand the table to meet the pasted Numbers selection range, keeping the Numbers cell coloration and formula results. Some Numbers cell formatting choices may not map to what Pages offers as the table support is different in the latter.

If you need to show a larger Numbers table in Pages, and where the page width and automatic column resizing won't work for you, then you should export the Numbers table selection range to a PDF, and then paste that PDF into Pages (without the Pages table). You can then rotate that PDF so it fits on the vertical page. Pages does only supports one document orientation throughout the document so no mixture of Portrait and Landscape is supported.