许多 (xǔduō) is a numeral meaning 'many, a lot of' — it directly modifies nouns like 许多人. Unlike 多 (duō, 'many/a lot' as adjective or predicate), 许多 cannot be used after numbers or as a predicate (*人许多). In everyday speech, 好多 (hǎo duō) is more common, while 许多 is neutral. For formal or written contexts, especially with abstract nouns, use 诸多 (zhūduō).