Many battles are mentioned in the Bible. But the largest army assembled was that of Zerah the Ethiopian.
فَقَدْ أَتَى لِمُحَارَبَتِهِ زَارَحُ ٱلْحَبَشِيُّ وَمَعَهُ جَيْشٌ مِنْ ٠٠٠‏,٠٠٠‏,١ جُنْدِيٍّ ،‏ أَكْبَرُ جَيْشٍ مَذْكُورٍ فِي ٱلْكِتَابِ ٱلْمُقَدَّسِ .‏