Arthashastra
तस्य द्वि रथ उत्तरा वृद्धिर् आ एक विंशति रथाद् इति ॥
The number of chariots in this array (of three groups of three chariots each) may be increased by two and two till the increased number amounts to twenty-one.
english translation
tasya dvi ratha uttarA vRddhir A eka viMzati rathAd iti ॥
hk transliteration by Sanscript