I would select everything in the card you created and on the Arrange panel, click Group. Then press cmd+D to duplicate that card and drag adjacent to the original card. Since both cards on the row are already grouped, you can select both and group that row.
Then press cmd+D to duplicate that row and move the copy down to the second row position. Repeat downwards until your sheet of business cards is complete. Do save your work… 😉
The issue with creating a sheet of business cards in Pages is that the application has no registration adjustments to exactly print your cards on a business card sheet. The better tool for this process is the free, online Avery Design & Print tool.
Another alternative is to create a single, high resolution (300 dpi) business card in (e.g. Affinity Designer) where all text is converted to vector via converting to paths, group all of the card layers and export as a PDF/X-4 document bearing only the card image. You now have a print-ready PDF that you can send to a custom online printer to generate 15-pt Index business cards for under $50. These will be stiff enough to pick locks and thwart the miscreants that like to fold and bend the cards.