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.
much / a lot
'viel' and 'massenhaft' both describe a large amount. 'viel' is the normal, neutral word and fits almost everywhere. 'massenhaft' is stronger: it means that something is present or occurs in a very large quantity. It often sounds somewhat emphatic or even negative, for example with problems, mistakes, or complaints. If you only want to say that the amount is large, 'viel' is enough. If the amount seems especially large, 'massenhaft' fits better.

Nach dem Bericht kamen massenhaft Nachrichten von Kunden.
After the report, huge numbers of messages came in from customers.
Zum Üben brauchen wir heute viel Papier.
We need a lot of paper for practice today.