I’ve practiced thousands of times.
Suddenly, thousands of bacon strips dropped from sky.
Victims of the titan attack numbered in the thousands.
What thousand? Could be five thousand, two thousand, or ten thousand, since you're asking. Let's generalize. How do you say all of those amounts at the same time? You say "thousands."
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.