Several parts combine into one character.

Compound ideograph: 夕 (evening) stacked on top of 夕 (evening) — two evenings, two days, time accumulating. The doubling principle (same as 林 = 木+木, 森 = 木+木+木 from earlier batches) applied to time: when nights pile up, things become "many". Identical across 繁體 / 新字体 / 简体.

Mandarin: duō, level 1st tone. 多 is one of the most foundational quantity-words in Mandarin and shows up in essential question/exclamation patterns: — 多少 (duōshao): "how much / how many" — the basic quantity question. 多少钱? (duōshao qián?) = "how much money / how much does it cost?" — 多么 (duōme): "how much / how very" — exclamation. 多么美! (duōme měi!) = "how beautiful!" — 很多 (hěn duō): "very many / a lot" — basic quantity. — 多 (duō) before age/time: "more than". 三十多岁 (sānshí duō suì) = "thirty-something years old".

Japanese: on-reading タ (ta) for Sino-Japanese compounds — 多数 (tasū, majority / many people), 多様 (tayō, variety / diversity), 多少 (tashō, somewhat / more or less), 多分 (tabun, "probably / maybe" — literally "much portion"). 多分 is one of the all-time most useful Japanese adverbs — it covers everything from "probably" to "maybe" in casual conversation. Kun-reading おお.い (ō.i) is the everyday adjective — 多い (ōi, many / numerous). Note carefully: 多い describes quantity (many things); 大きい (ōkii) describes size (a big thing). They are NOT interchangeable, despite both starting with おお.

Memory aid: two evenings stacked — accumulated nights become "many".