number one
壹 is a formal or legal variant of 一, meaning the number one. It is used in contexts like financial documents to prevent forgery, but its core idea is simply the numeral one.
壹 is an ancient formal variant of 一 (one), likely developed to prevent alteration in documents. Its complex structure may have been chosen for its resistance to tampering, but the exact historical origin of the components is uncertain.
The top 士 looks like a samurai standing guard over a covered pot 冖 and a bean 豆, protecting the single precious bean inside—just one.
For イチ, imagine an itchy samurai (士) guarding the single bean: itchy -> イチ, and he can't scratch because he's holding the number one.