It depends. The simple answer might be the minimum amount you need to achieve your educational goals.
How much you should borrow depends on many factors including:
Cost of attendance
Loan limits established by the federal government and other lenders
Your existing financial commitments
Other resources you may have
The amount of debt you can afford to repay once you graduate
Current federal regulations limit your borrowing to the total cost of attendance (as determined by the financial aid office), less other aid you might be receiving, such as scholarships, grants, or work study. Cost of attendance typically includes tuition, books, fees, room and board, and other miscellaneous personal expenses.
If you are able to borrow less than the amount suggested in your award letter from your school, this will be to your advantage. However, if you find that you need more than the school has allotted, you have the right to appeal the decision if you have not reached the maximum amount as established by federal regulation.
The federal government and other lenders place annual and aggregate loan limits on individual student borrowers. The aggregate limit means that each borrower, in the span of his/her education, cannot borrow more than this total amount.
ASSESS CURRENT FINANCIAL STATUS
Carefully and honestly assess your current financial status and any financial commitments you've made prior to entering graduate or professional school. If you have undergraduate student loan debt, keep in mind the total dollar amount owed and the interest that has accrued. Also factor in consumer debt, such as auto loans or credit cards, and understand the repayment obligations of each. Your education loans are not meant to cover these prior obligations.
Another consideration is a realistic determination of your future income. Research the job market and starting salaries in the field you plan to pursue. Remember that you will be paying for your education with your future income. Make sure that your monthly loan payments will not prevent you from paying your living expenses.