"They see a number of birds on the house" -> should be accepted, submitted report.
"They see a number of birds on top of the house" -> should be accepted.
"They see some birds on the house" is a much more natural way to say this in English.
But the two are contrasting statements. "A number of" is no the same as "some." If you wre to poll a group of English speakers, "a number of" would be perceived as more than "some." I imagine its the same with Indonesian.