Usage of 'incredible'

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 .

an incredible amount/number

So it has to do with ‘‘of’’ ?

With a number/an amount

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

100 can be seen as an amount.

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

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 ?