die Legion
legion
A large independent military unit in the army of ancient Rome, consisting of about 3,000 to 6,000 soldiers.
Caesar führte seine Legionen erfolgreich durch Gallien.
Caesar successfully led his legions through Gaul.

legion
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legion
A large independent military unit in the army of ancient Rome, consisting of about 3,000 to 6,000 soldiers.
Caesar führte seine Legionen erfolgreich durch Gallien.
Caesar successfully led his legions through Gaul.

You can recognize the meaning of 'Legion' (legion) from the context. If it is about ancient Rome, it refers to a military unit. In modern contexts, it can mean an elite unit (e.g. Foreign Legion) or, usually with 'von' (of), a huge number.
Die römische Legion marschierte tagelang.
The Roman legion marched for days.
In the figurative meaning 'große Menge' (large number), 'Legion' (legion) is almost always used with the preposition 'von' (of) and a plural noun. The structure is: 'eine Legion von [etwas/jemandem]' (a legion of [something/someone]).
Im Sommer kämpfen wir gegen eine Legion von Mücken.
In summer, we fight against a legion of mosquitoes.
Using 'Legion' (legion) for a large number is figurative and belongs more to written language or an elevated style. It sounds more expressive than simply 'viele' (many).
Er musste sich durch eine Legion von Vorschriften und Formularen kämpfen.
He had to struggle through a legion of regulations and forms.
In a historical context, 'Legion' (legion) is often used with verbs such as 'anführen' (to lead), 'kommandieren' (to command), or 'aufstellen' (to form/raise). Soldiers could 'dienen' (serve) or 'kämpfen' (fight) in a legion.
Der Feldherr führte seine treueste Legion in die Schlacht.
The general led his most loyal legion into battle.
amount / crowd
'die Menge' and 'die Legion' can both express that there are very many people or things. 'die Menge' is the normal, neutral word for an amount or quantity. In this meaning, 'die Legion' is figurative and sounds stronger, more vivid, and often a bit more elevated. It is used when you want to emphasize that something is almost too numerous to take in. In everyday language, 'Menge' is usually more natural; 'Legion' fits better in more expressive sentences.

Nach der Sendung bekam der Verein eine Legion von Nachrichten.
After the broadcast, the organization received a legion of messages.
Im Supermarkt achtet Maria auf die Menge Zucker in den Getränken.
At the supermarket, Maria pays attention to the amount of sugar in the drinks.