To generate more sheets:

Make sure you have a sheet open, then choose Sheet > Randomize New Problems. Mathemania will wipe the sheet clean and generate new problems from the same numeric ranges.

If you would rather keep the exact same problems (perhaps to guarantee fairness), you can choose Sheet > Reshuffle Existing Problems. This will keep all the same problems, but mix up their locations on the page.
After randomization, you can print the new sheet using File > Print. But this could easily become tedious.