无数 (wú shù) means 'countless, numberless' and emphasizes that something is too many to count. It differs from 无限 (wú xiàn), which means 'unlimited, unbounded' and often describes abstract limits (e.g., potential, space). 成千上万 (chéng qiān shàng wàn) is a more vivid phrase meaning 'thousands upon thousands', often used for concrete groups of people or things.