You are allowed one exemption for each person you can claim as a dependent.

According to the IRS Frequently Asked Questions, you are not allowed a dependency exemption for a stillborn child:

Question: My child was stillborn. He died right before he was delivered. Can I claim my child?
Answer: You cannot claim a dependency exemption for a stillborn child.

Additional information, per Publication 501 Exemptions, Standard Deduction, and Filing Information:

Death or birth of child. A child who was born or died during the year is treated as having lived with you all year if your home was the child’s home the entire time he or she was alive during the year. The same is true if the child lived with you all year except for any required hospital stay following birth.

Child Born Alive. You may be able to claim an exemption for a child who was born alive during the year, even if the child lived only for a moment. State or local law must treat the child as having been born alive. There must be proof of a live birth shown by an official document, such as a birth certificate. The child must be your qualifying child or qualifying relative, and all the other tests to claim an exemption for a dependent must be met.

Stillborn child. You cannot claim an exemption for a stillborn child.