Translation:There are ten birds on top of the car right now.
只 is always necessary i when talking about any quantity of nouns (animals) but the rules change depending on certain criteria. Such as 條 in regards to fish or 輛 in regards to cars. It's similar to how English has different words for animals in groups; ie. School of fish, herd of sheep. Etc.
上 is an "prepositional word." It means on top of something (Although, in previous lessons, it has many other definitions). 里 is a prepositional word meaning "in" or "inside of." So, 里 would be used to say that something is inside .
「现在汽车里有十只鸟。」 "There are now ten birds in the car."
「现在汽车上有十只鸟。」 "There are now ten birds on top of the car."
There are other prepositional words such as 外 (outside of) and 下(under).
「现在汽车外有十只鸟。」 "There are now ten birds outside the car."
「现在汽车下有十只鸟。」 "There are now ten birds under the car."
Couldn't it be "on a car" rather than "on the car"? Since Mandarin doesn't use determiners the same way we do in most Western languages, it seems that context would tell the difference, but that "a car" and "the car" should both be correct. If I'm wrong please explain the reason. Thank you.