On my OMB 1099R 2a shows Taxable Amount UNKNOWN. How much tax should I pay on this ?

The software will calculate the tax owed--if any.   "unknown" is there because the financial institution that issued the 1099R does not have any way to know what other taxable income you received.   

You will use the simplified method.  When entering your 1099-R, next to the taxable amount is a learn more link.  click on that, scroll down until you find the simplified method link. Click on that and follow the instructions.