号 began as a wail — a mouth 口 crying out (the fuller traditional form 號 adds a tiger 虎 roaring). From a cry to a call-sign to a label: the number or name by which something is called.
号 organizes Chinese life: dates (几月几号? — what date?), sizes (大号, large), phone numbers (手机号), and social media handles (微信号, your WeChat ID). 今天几号? is among the first questions every learner masters. In classical times a scholar's 号 was his literary pseudonym — Su Dongpo's 东坡 ('Eastern Slope') is a 号 more famous than his real name.
A mouth 口 calling out a label — NUMBER, DATE, SIGN.
Classical literati collected 号 (pen names) like modern people collect usernames — some poets had a dozen.