There were Germanic migrations that moved over the Rhine around the time of Caesar. The Germanic peoples began to move from Scandinavia around the second millennium BCE. From there they move to the west and east. The Germanic peoples in the west saw a landscape that was heavily forested. At that time and on to through the time of Caesar, the Germanic peoples were largely hunters and shepherds. Both the limited amount of food their lifestyle produced and restricted living space caused by the heavy forests of the West German landscape gave rise both to overpopulation and famine. These and the occasional political issues then pushed groups of west Germanic peoples further south and closer to the Rhine. Largely they pushed the Celts from Central Europe to the Rhine, but were only really halted in their advance by the Romans in the time of Caesar. However that did not stop them trying to migrate further south past the Rhine.

These group migrations could vary in size and their isn't a way of knowing how big they became for sure. We do know that the Germanic tribes would often band together through their migration to form larger groups such as the Cimbri and Teutoni mentioned in de Bello Gallico or even the army under Ariovistus. In the introduction of The Conquest of Gaul translated by F.P. Long, Cheryl Walker notes that ultimately there is suspicion that Caesar may have embellished numbers or Ariovistus' arrogance, but we ultimately lack the evidence prevents us for saying for sure that he did (xiv). Even if the numbers do seem unlikely.

Sources:

Bury, J.B. The Invasion of Europe by the Barbarians. New York & London: W. W. Norton & Company, 2000.

Caesar, Julius. The Conquest of Gaul. trans. F.P. Long. New York: Barnes & Noble, 2005.

Edit: fix formatting