Use of any number other than 1024 is mathematically incorrect, but here is a hint: there are 1025 values (endpoints) associated with an interval divided into 1024 segments.
You can't represent all of the endpoint values using 1024 numbers, you can represent only the 1024 ranges of values associated with each interval.
But if additional information, like how the ADC works, helps you to accept the correct answer, that is fine.