When you say "infant" are you referring to a child born in 2019 or 2020? If your child was born in 2019, you need the Social Security number in order to claim your child as a dependent on your 2019 return. And you should say the child lived with you the whole year.
If your "infant" was born in 2020, you will claim that child on your tax return next year when you file a return for 2020.
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