A light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). It's a unit of astronomical distance.

There are an enormous number of inches in a light-year, over 3.7 x 10^20 inches.

This is because a light-year is an incredibly vast distance, while ten million inches is a relatively small, human-scale measurement.

Inches are not typically used for measuring astronomical distances. Astronomers use units like light-years, parsecs, or astronomical units (AU) because they are more suited to the immense scales of space.