• 複数ふくすう — multiple; plural
• 複雑ふくざつ — complicated
• 複製ふくせい — duplication
• 重複ちょうふく — overlap; redundancy
• 複写ふくしゃ — copy; reproduction
• 複合ふくごう — compound; composite
• 複眼ふくがん — compound eye

複 combines 衣 (clothing) with 复 (phonetic). Its earliest sense was “layered garments.” By extension it came to mean overlapping, repeating, or compounded — ideas still preserved in words like 複数 and 複雑.

複数ふくすうの 人ひとが 集あつまりました。
Several people gathered.

彼かれの 説明せつめいは 複雑ふくざつすぎる。
His explanation is too complicated.