KanjiJLPT N4
Many

many, frequent, much

Mnemonic

Picture one evening sky stacked directly on top of another evening sky. The horizon doubles, then triples — suddenly there are many layers of sunset bleeding into each other, an infinite cascade of purple dusks you could never hope to count.
Additional thoughts
Two 夕 (evening) radicals sit one atop the other to form 多. Visualize evenings literally piling up — the more evenings stack, the more 'many' they become. Quantity born from repetition.
Quick recall
Evening stacked on evening creates many layered sunsets.

Details

The keyword for 多 is many. This kanji conveys the concept of a large number or quantity, indicating abundance or plurality. It is one of the most fundamental characters for expressing that something exists in great amounts or that there is more than what might be expected. It stands in contrast to the idea of few or scarce, emphasizing excess or multiplicity.
On'yomi
Kun'yomi
おお.い、まさ.に、まさ.る