Can i take 20 mg of provera (medroxyprogesterone) for 5 days instead of 10mg for 10 days ?

No: You should take medication as prescribed. 10mg of provera (medroxyprogesterone) is the max daily dose.

Not sure: Doubling the dose by reducing duration to half may not be safe strategy as such medications are start at the lowest dose by health care providers to monitor how well that dose is working for you. You and your health care provider should then decide regular dose for you and talk regularly (every 3-6 months) whether you still need treatment with PROVERA (medroxyprogesterone) or require any adjustment in dosage.