Because I have looked up bus fleet sizes before and 400 is a good number for a smaller city. The E12PRO they are buying seats 40 and has 30 standing places, so a capacity of 70 people if full. 400 buses * 70 people capacity = 28,000 people, which can be transported in those buses at the same time. For a small city that is plenty.

China has about 900,000 buses and a population of 1.4billion so a bit less then 0.7 buses per 1000 people. So the 400 buses would be enough for 570,000 people, which is a pretty decent size city.