One Thousand
numeral
If you have one thousand, you have... one thousand. Simply saying 千 is the most common way to say 1,000, but sometimes you might use 一千 to emphasize or clarify the exact number.
いっせん
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.
一千くらいです。
It's about one thousand.
このトイレットペーパー、一千ドルもしたんだって！
Apparently, this toilet paper was 1,000 dollars!
一千フィートの山の上でキャンプをした。
We camped out on a 1,000-foot mountain.