तस्य द्विरथोत्तरा वृद्धिरैकविंशतिरथादिति ॥ कZ_१०.५.१५ ॥
tasya dvirathottarā vṛddhir aikaviṃśatirathād iti
その(標準陣形)では、増加は戦車を二両ずつ加えることで行い、二十一両から始める。
Even-number increments preserve bilateral symmetry (left/right) and keep wings balanced while scaling the formation.
A minimum viable baseline size for this formation family, from which standardized expansions are authorized.