The maximum amount of loans you can take out per year is determined by the college or university you are attending. The "cost of attendance" is determined by the financial aid office which includes tuition, fees, and an estimated amount for living expenses (rent, books, car payments, expenses, insurance, etc.). The maximum amount you can take out in Stafford loans each year (if you are in professional school) is $40,500 -- of this, $8,500 will be subsidized (meaning that the government pays the interest on that balance of the loan while you are in school) and the remainder, or $32,000, will be unsubsidized and interest will accrue on that amount while you are still in school.
If you need more money than the $40,500 provided by the Stafford loans, you can apply for other loans out there (Sallie Mae probably offers the best rates on private student loans) BUT you can only get up to the amount of the cost of attendance as determined by the school.
For example: If you have a $2,500 scholarship, $40,500 in Stafford loans, and your cost of attendance is determined to be $50,500, you can get an additional $7,500 in private student loans.
Private student loans accure interest while you are in school, and they are not a locked-in rate meaning that they can adjust to very high rates, especially if you miss a payment. Thus, private loans should be used sparingly (if at all).
Hope this helps.