How can the normal reader understand the numbers found in the Bible? The answer is quite simple if the Old and New Testaments are read carefully, namely, by how numbers were used and by understanding that they were used in textual and cultural contexts by those ancient societies with which Israel lived and interacted. Hebrew and Greek letters were used to map to numbers.
It is fascinating to understand how the Biblical writers used numbers! The Bible authors used numbers to count things and people. They used them for weights, measures, and time. They were familiar with arithmetic: Addition (Genesis 5:1-31; Numbers 1:20-46), Subtraction (Genesis 18:26), and Multiplication (Leviticus 25:8). Also, the frequent use of fractions shows a clear understanding of division: Half (Exodus 24:6); One Quarter (Nehemiah 9:3; Revelation 6:8); One Fifth (Genesis 47:24); One Tenth (Numbers 18:26).
The Bible has many references to large numbers too! Indefinite numbers are also used in the Bible quite clearly. Very high numbers like a million, ten thousand times ten thousand, thousands of thousands, and hundreds of millions are mentioned too.
God is a supreme being who is transcendent and immanent. This means that He knew the limitations of the knowledge of the Bible writers vis-a-vis Mathematics (in the context of the modern form with symbols, equations and operators to mention just a few attributes). In His omniscience, God provided proper mappings to numbers and counting (in Hebrew and Greek ) that could be used by the ancient people of the Old and New Testaments.