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At the time of the fall of Rome it was around ~12 million, around 8-9+ million at the time of Charlemagne (350 years later) and down to a record low of ~6-7 million a few decades later (with the Carolingian empire shattering and invasions/pillagers coming in again), which is pretty much the population level we had during the Bronze age. It never got lower and after that the demography starting going up pretty fast with the record from Roman time broken during the XIth or XIIth century (thanks to stable monarchy, monachism spreading and greatly improving the land and agriculture etc.).
Nota bene : I'm using numbers from Histoire de la population française by Jacques Dupâquier. It's a bit old now (1988) but it still serves as a references. There have been more recent estimations based on archeology that give a number of around 12-14 million inhabitants in Gaul during the century before the Roman invasion.
Estimations of Frankish immigration range from 100 000 to 350 000. So it was indeed pretty negligible. Like most of those migrations, there are only traces in places the really concentrated. But it's their political and cultural impact that matters anyway.
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