I noticed that the word heaven is both singular and plural in scripture. Could you please explain?

There are in the Bible 551 references to the word: "heaven", and there are 127 references in the Bible to the word "heavens". There are three "heavens" found in the BOOK.

The first heaven is the firmament. This heaven is the closest to the Earth.

Genesis 1:8And God called the firmament Heaven. And the evening and the morning were the second day.
See All... “And God called the firmament Heaven. And the evening and the morning were the second day.”  The Bible also, refers this heaven as the sky.

Matthew 16:2He answered and said unto them, When it is evening, ye say, It will be fair weather: for the sky is red.
See All... “He answered and said unto them, When it is evening, ye say, It will be fair weather: for the sky is red.”

The second heaven is the space is where the sun, moon and stars are located.

Deuteronomy 4:19And lest thou lift up thine eyes unto heaven, and when thou seest the sun, and the moon, and the stars, even all the host of heaven, shouldest be driven to worship them, and serve them, which the LORD thy God hath divided unto all nations under the whole heaven.
See All... “And lest thou lift up thine eyes unto heaven, and when thou seest the sun, and the moon, and the stars, even all the host of heaven, shouldest be driven to worship them, and serve them, which the Lord thy God hath divided unto all nations under the whole heaven. The plural form of “heavens” quite often is in reference to the stars beyond this solar system: the myriad of galaxies in the universe.

Job 26:13By his spirit he hath garnished the heavens; his hand hath formed the crooked serpent.
See All... "By his spirit he hath garnished the heavens; his hand hath formed the crooked serpent."

The third heaven is the location of God's throne: situate to the utter most north from the earth. According to the Apostle Paul he was caught up to the third heaven.

2 Corinthians 12:2I knew a man in Christ above fourteen years ago, (whether in the body, I cannot tell; or whether out of the body, I cannot tell: God knoweth;) such an one caught up to the third heaven.
See All... “I knew a man in Christ above fourteen years ago, (whether in the body, I cannot tell; or whether out of the body, I cannot tell: God knoweth;) such an one caught up to the third heaven.”

The location of this heaven “in the sides of the north” is shown in Scripture in Isaiah 14:13For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:
See All... in the rebellion of Satan. "For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:”