Now take assumptions that have both a 3 and a C on them. Let's call these the Most Riskiest assumptions.

If you have more than 5 assumptions that are both your weakest assumptions, and would have the highest impact if they turn out to be bad assumptions, then just pick the assumptions with 3 and C on them and repeat exercise. For example you should now have post it notes with 3.1,3.2 or 3.3 likewise C.1, C.2 or C.3. Rewrite the assumptions if necessary.

This should certainly bring you down to your 2-5 Most Riskiest Assumptions.