Genesis 1
6 ¶And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. (*)
7 And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so.
8 And God called the firmament Heaven. [plural in the Hebrew -- Heavens] And the evening and the morning were the second day. (*)
9 ¶And God said, Let the waters under the heaven be gathered together unto one place, and let the dry land appear: and it was so.
10 And God called the dry land Earth; and the gathering together of the waters called he Seas: and God saw that it was good.
. . .
14 ¶And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years: (*)
15 And let them be for lights in the firmament of the heaven to give light upon the earth: and it was so.
16 And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also. (*)
17 And God set them in the firmament of the heaven to give light upon the earth,
18 And to rule over the day and over the night, and to divide the light from the darkness: and God saw that it was good.
19 And the evening and the morning were the fourth day. (*)
20 And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven. (*) (*) (*) (*)
. . .
23 And the evening and the morning were the fifth day. (*)
Just one firmament. Not just one heaven, but definitely just one firmament.