expression, adverb, noun which may take the genitive case particle 'no'
in great numbers; many; numerous
Used adverbially or before の to say that people or things exist, appear, are made, are reported, etc. in large numbers. It emphasizes countable abundance rather than amount in general.
Many researchers from overseas also attended the conference.
この地域では古い寺が数多く残っている。
Many old temples remain in this area.
たくさん is the casual, conversational word for 'a lot', whereas 数多く is more formal and typical of written or explanatory language.
多数 is a noun/na-adjective meaning 'a large number' or 'majority', while 数多く works adverbially or before の to describe things existing in great numbers.
数々 highlights the variety among many things (e.g. numerous different achievements), while 数多く simply stresses the sheer quantity.
Transparent compound of 数, 'number', and 多く, the adverbial form of 多い, 'many'; literally, 'many in number.'