I did not suggest that they have twenty vehicles either. I replied on a different post, but an average school bus route in the US is about 32 mi[0]. Zum buses are capable of 155 mi[1]. Given their "up to" language, I'd like to think that they actually offer smaller battery capacities as well, but especially for a non-rural district like Oakland[2], if one were to carry 155 mi range for a typically 32 mi or less route, it would be quite an overkill.
One can have a few 155 mi range vehicles for field trips, sports games, etc. but the majority of the fleet can be much smaller for every day uses.