What’s the most I would have to repay the IRS?

That depends on what your actual annual income turns out to be. If your income goes over 400% FPL you will have to repay the full advance premium tax credit amount you received. If your actual annual modified adjusted gross income is higher than what you projected but less than 400% FPL, there are repayment limits based on income. On your tax return, you will compare the actual amount of advance premium tax credit you received during the year to the amount you should have received based on your modified adjusted gross income, and then pay back the excess up to the repayment limit.