Step 1: Read the given sentence carefully.
“How many stars are there in \underline{\hspace{2cm}} sky?”
Step 2: Identify the noun sky, which refers to something that is common and unique for everyone.
Step 3: For unique objects (such as the sun, the moon, the earth, and the sky), the definite article ``the'' is always used.
Step 4: Therefore, the correct sentence is:
“How many stars are there in the sky?”