Your warranty should define how many dud pixels you can have.
No, if your TV is a DLP then it doesn't have pixels, but instead a DLP chip that has likely gone faulty. You should call Mitsubishi customer service and explain to them the issue you're experiencing. Generally, they're warrantying the chip, but typically charging $200 for the repair. Many have claimed the ability to get the fee waived with a little bit of polite firmness. See the linked lengthy forum post for more details.