Answer:
It is pronounced ichiman issen. In hiragana, it is いちまんいっせん。
The ichi before sen changes to いっ. The small tsu is silent, but strengthens the following consonant.
However, you only say 1000 as issen when it’s following man. If it is by itself, you just say sen. (That is, in Japanese, just say “thousand” when it’s just “one thousand” all by itself—you don’t normally need to say “one” in that case.)
Here’s how to pronounce all the thousands:
1000 sen (or issen if it’s after a larger number such as ichiman)
2000 nisen
3000 sanzen (notice the change of s to z)
4000 yonsen (not shisen!)
5000 gosen
6000 rokusen (the u is almost silent in most dialects)
7000 nanasen
8000 hassen (change is similar to 1000)
9000 kyūsen (lengthened u)
