Nope, you can borrow up to the entire cost of attendance as set by your school, and it can all be in federal loans! You'll be approved for likely a mixture of Direct and Grad PLUS loans. If you need more money than that to subsist during school (for example, if you're a single parent with dependents) you may need to take out private loans, which I think (?) might be income-based, but that's a relatively rare situation.