If by "sheet" you mean sheet in the Numbers sense (the "tab", not a table with the rows and columns) then you can copy-paste your model sheet anywhere by going to the sheet name in the tab at the top and choosing Copy Sheet from the dropdown when you click the "v". The select another sheet (even in a different document), go the dropdown by the name in the tab at the top of any sheet and choose Paste Sheet.
If you're creating similar sheets in the same document you can just choose Duplicate in that same dropdown menu.
If, on the other hand, you mean you have set up a table the way you like it, and want to create others just like it, then just click in it, click the concentric circles "bulls-eye" upper left, command-c to copy, click the canvas, and command-v to paste.
Or, if you mean you have set up a document the way want it and will want to create similar documents with the same setup, then you can go to File > Save as Template ... and give that a name. Then to start a new document just like it, File > New and choose your template in the template chooser. It will be in the 'My Templates' section.
SG