I am on H1B visa and my wife is on H-4 (she can not work), both from India. My tax withholding on W-4 is based on single status. I want to file a new W-4 changing it to married status. How many allowances should I claim?

Under special IRS rules for foreign nationals, the MOST allowances you can claim is TWO.

This is done to forceover-withholding to ensure that you PAY the taxes that are due and to give you an incentive to file your U.S. tax return after you return to your home country.