Usage of 'incredible'

[duc]

Please look at this:"The factory produces an incredible 100 cars per hour.''
I saw this in Longman dictionary and wondered why they use ''an'' here because ''100 cars'' is plural.
Please explain it for me.
Thanks in advance .
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Don't see your shade think you are great

[Molly]

an incredible amount/number

[duc]

So it has to do with ''of'' ?

[Molly]

[duc]

I mean ''a number/ amount of''(but I still don't think they are so careless to make that mistake)

[Molly]

[duc]

Oh,thank you very much.First I misunderstood your explanation but now I make it out very clearly.

[duc]

And I want to ask whether this sentence :"The factory produces an incredible 100 cars per hour.'' sounds natural to you.
Some help of native speaker,please ?