The reading is a little bit weird, so be careful. You should already know the on'yomi readings, but the ひゃく rendakus to びゃく. This pattern will repeat itself with 三 + many counter kanjis that can rendaku, so pay attention for it as you read Japanese.
I have three hundred manga.
When I was taking a nap, somebody rang the doorbell 300 times.
One year is three hundred and sixty-five days.