reihenweise
in large numbers
In large numbers and often one after another; en masse.
Nach der schlechten Nachricht gingen die Kündigungen reihenweise ein.
After the bad news, resignations came in in large numbers.

in large numbers
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in large numbers
In large numbers and often one after another; en masse.
Nach der schlechten Nachricht gingen die Kündigungen reihenweise ein.
After the bad news, resignations came in in large numbers.

The adverb 'reihenweise' is made up of 'Reihe' (row/line) and 'Weise' (way/manner). It literally means 'in the manner of a row' and expresses that something happens very often or in large quantities, as if it were standing in a long line.
Die Touristen kauften reihenweise Souvenirs.
The tourists bought souvenirs in large numbers.
As an adverb, 'reihenweise' can stand flexibly in the sentence. It often stands at the beginning of the sentence in position 1 to emphasize the large quantity. The verb then follows directly.
Reihenweise fielen die Schüler durch die schwere Prüfung.
Students failed the difficult exam in large numbers.
'Reihenweise' is often used with negative or unexpected events to describe their mass occurrence. Typical phrases are 'reihenweise ausfallen' (to be canceled/fail in large numbers), 'reihenweise Fehler machen' (to make lots of mistakes), or 'reihenweise krank werden' (to fall ill in large numbers).
Wegen des Streiks fielen die Züge reihenweise aus.
Because of the strike, trains were canceled in large numbers.
individual / separate
'einzeln' and 'reihenweise' are clear opposites. 'einzeln' means that people or things occur separately from one another or are dealt with one after another. It can also mean that there are only a few. 'reihenweise', on the other hand, means that very many occur, often one after another or in large numbers. If just one thing happens after another, 'einzeln' fits. If many similar things happen, 'reihenweise' sounds more natural.

Wegen der Grippe fehlen in dieser Woche reihenweise Schüler.
Because of the flu, large numbers of students are absent this week.
Bitte legt die Bücher einzeln auf den Tisch, damit ich sie zählen kann.
Please place the books on the table one by one so I can count them.