zahlreich
numerous
Present in large numbers; many.
Zahlreiche Besucher kamen zur Eröffnung des Museums.
Numerous visitors came to the opening of the museum.

numerous
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numerous
Present in large numbers; many.
Zahlreiche Besucher kamen zur Eröffnung des Museums.
Numerous visitors came to the opening of the museum.

The word is made up of 'Zahl' (number) and 'reich' (rich). So it literally means 'rich in number' and describes a large quantity.
Das Land ist reich an Bodenschätzen.
The country is rich in natural resources.
'Zahlreich' (numerous) can stand not only before a noun but can also be used as an adverb. It then means 'in large numbers' and often appears at the end of the sentence.
Die Fans erschienen zahlreich zum Konzert.
The fans showed up in large numbers for the concert.
'Zahlreich' (numerous) is used more often in written language or in more formal contexts. It often sounds somewhat elevated.
Zahlreiche Studien belegen die Wirksamkeit dieser Methode.
Numerous studies demonstrate the effectiveness of this method.
much / a lot
'viel' and 'zahlreich' both describe that a large amount is present. 'viel' is the normal, very general word and fits especially well with uncountable things like work, money, or time. 'zahlreich' is used more for things or people that can be counted, for example visitors, questions, or examples. It sounds somewhat more formal than 'viel'. The words can be confused because both express 'a large amount'. A practical tip: with 'viel Wasser' (a lot of water) or 'viel Arbeit' (a lot of work), you do not say 'zahlreich'. With 'zahlreiche Besucher' (numerous visitors), you mean many individual people.

Nach der Präsentation hatten die Schüler zahlreiche Fragen.
After the presentation, the students had numerous questions.
Vor der Reise bleibt nicht viel Zeit zum Packen.
Before the trip, there isn't much time left to pack.