thousand
仟 centers on the number one thousand. It is a variant or specialized form used in certain contexts to represent a thousand, often in formal or historical writing.
仟 combines 亻 (person) with 千 (thousand). It is likely a specialized character created to represent a thousand people or a leader of a thousand, but the exact historical development is uncertain.
A person 亻 beside the number 千: imagine a leader standing before a thousand followers, representing the idea of a thousand.
For セン, picture a thousand people each holding a 千 (せん) yen bill: 千 -> セン, and the person radical 亻 stands for the leader of that thousand.