There isn't a fixed ampacity, per se, for copper busbars as it depends on way too many variables (as peggus just pointed out).
Of course, currents in both the motor and battery loop vary wildly in EV applications, even during highway driving, which makes it even harder to nail down the required cross-sectional area for the conductors.
That said, the (extremely conservative) rule from the NEC is 1.55A/mm² (1000A/in²) for pure copper busbar. This rule results in a rather low 30C rise above ambient, when a 60C or even a 90C rise might be perfectly acceptable in a given (non-NEC) application.
That said, this is a good sizing rule for the expected continuous current (e.g. - 300A on the motor side with the typical 9" advanced timing motor), as relatively long overloads of twice that will be perfectly acceptable (causing a 60C rise) while briefer overloads of 10x or even 20x that amount will be ok, too.
The ampacity of a busbar is ultimately limited by the allowable temperature rise (which can't be any higher than the melting point of copper). Consequently, a wider but thinner bus bar will have a higher ampacity than a thicker bus bar with the same cross-sectional area because the former has more surface area to shed heat.
