I would like to know what kind (thickness,etc.) of insullation is needed to put up in our garage ceiling which is the floor to the bonus room ? We have R 16 left over (but can be returned) and wondered if that can be used or should we get something else? Thank you !

The R-value of the insulation that should go in the ceiling of the garage is the same as in the floor. Where I live (SC), it's generally R19.