This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. The only thing that's a bit weird is the reading of the 一. The ち turns into a small っ, making the reading for this word いっせん.
Ten million people gathered at the live show.
That katana is one thousand yen.
I farted at an elevation of 1000 feet.