[JKT]

That should be automated - I would send feedback to OmniGroup to request that this feature be added using the Help>Send Feedback menu item in OmniGraffle.

In its absence, there is a workaround. If you set up your Master canvas so that it has the same format as your Canvasses with additional pages (e.g. say it is 2 pages wide and 5 deep to give you your 10), and add the variables to each page on that Master canvas, when you then create a new canvas (and extend it to that page format) the variables will show up automatically.

In fact, the simpler thing to do is to create a Master canvas with pages that exceed the number you are likely to ever use and add your variables to each page on that Master canvas. When you create a new canvas and it contains less pages, you just need to reduce it down. If, for any reason, you ever need to go beyond the bounds of the Master, you can also just add the variables to the additional pages by hand and they will auto-populate with the correct numbers and any other canvasses present will auto-correct their own entries.