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. Just take note that 千 is read as ぜん and not せん because of rendaku.
I’ve practiced thousands of times.
Suddenly, thousands of bacon strips dropped from sky.
Victims of the titan attack numbered in the thousands.
How many thousand yen?