"This bus has a lot of passengers."
Translation:Bus ini banyak penumpangnya.
I've gathered that it is expressing it in a way like "this bus is multi-passengered" which translates more naturally into this bus has many passengers.
Banyak alone means (adj.) many, but banyak with something behind it makes it into a modified adjective. Banyak operates a bit different to English's many. You'll find this is a couple of languages where people say "These things are many" rather than the English way which is there are many of these things.
I figure it is translating the meaning into more natural English, but the way it is structured is as quoted above.
Hopefully my ill worded explanation is of some help.