What is the max loan amount for a credit score of 620?

The max loan to value amount based on your credit score would be 100% financing. As far as loan amount that would be based off your income and monthly obligations, credit score dictates how much of a percentage you may borrow and your interest rate, it doesn't dictate how much money you can borrow. There are also alternative means of documentation and mortgage programs if you do need to qualify for more money.