
massenhaft
in large quantities
Present or occurring in a very large quantity or number.
Nach dem Sturm lagen massenhaft Äste auf der Straße.
After the storm, there were branches lying in large quantities on the road.
massive / in large quantities
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in large quantities
Present or occurring in a very large quantity or number.
Nach dem Sturm lagen massenhaft Äste auf der Straße.
After the storm, there were branches lying in large quantities on the road.
The word is made up of 'die Masse' (a large, unspecified quantity) and the ending '-haft' (-like). So it literally means 'in the manner of a mass'.
Im Herbst gibt es massenhaft Pilze im Wald.
In autumn, there are mushrooms in large quantities in the forest.
'Massenhaft' is very often used as an adverb and then remains unchanged in the sentence. It then means 'in large quantities'.
Für das neue Produkt gab es massenhaft Vorbestellungen.
There were huge numbers of preorders for the new product.
The word belongs more to colloquial language and is often used for emphasis or exaggeration. It sounds stronger and more informal than 'sehr viele' (very many).
Auf der Party gab es massenhaft zu essen.
There was tons of food at the party.
numerous
Zahlreich and massenhaft can both describe a large number of people or things. Zahlreich mainly emphasizes that many individual units are present and sounds more neutral; massenhaft emphasizes the very large quantity and the sheer scale of their occurrence more strongly. Zahlreich fits a factual statement, while massenhaft is more appropriate for an overwhelming quantity.

Massenhaft Gäste warteten vor dem kleinen Eingang.
Huge numbers of guests were waiting outside the small entrance.
Zahlreiche Gäste warteten vor dem Eingang.
Numerous guests were waiting outside the entrance.