The correct answer is: 1024 (but the error is so small if you use 1023, it actually doesn't matter since the reference volt will vary with a much larger margin of error than your calculation will introduce). But in case you are still wondering why not 1023, here it is. After reading many responses…

Dividing by 1023 or 1024? The final verdict on analogRead

Robin2 March 25, 2018, 3:03pm 4

And you can divide by 1024 with >>10

...R