ten, hundred and thousand...

There is a convention in Indian thought especially in Sanskrit that when a large number of things are being reported , and especially when that is not easily countable then that count is denoted by a sufficiently huge number like thousand or lakh..

This is especially true when such number is huge for that particular class of material..

See in Vedic Purusha Sooktham

Thousand eyes, thousand heads, etc are indeed huge for eyes or heads..

सहस्र शीर्षा पुरुषः सहस्राक्षः सहस्रपात् Shasra Sheershaa purushaH sahasraakshaH sahasrapaad 

The Purusha of thousand heads in purusha sooktham पुरुष सूक्तम् just describes a being having a large number of heads which are not easily countable..

The number need not be exact..

We cannot say that about dasha दश.. ten or Vimshati विंशति.. 20.. because ten and twenty are easily countable

but in certain categories even shatam शतं or hundred can be considered as huge..

This is the general interpretation given by commentators of Vedas, and scholars of Sanskrit literature too..