Thank you for encouraging your child to read nightly. There is no REQUIRED number of books for first or any grade.
Reading involves hundreds of small skills integrated to help correctly pronounce words and to comprehend the meaning.
Each first grader begins at his/her own level. A book is appropriate if help is needed on only one or two words in the book. Stopping to sound out words or ask for help on sight words slows down enjoyable independent reading.
A reasonable time expectation for early elementary is about ten to fifteen minutes of reading in a quiet environment.
How are we doing??? From my perspective, I've noticed improved word recall and expression during the first four weeks of school Most pupils have read 60 books or about three short books a day.