We're not sure. We do know that the proportion of Italians in the legions decreased rapidly over the course of the first century AD, and that already by the end of the first century BC non-Italians were becoming a significant minority. Brunt's estimate that by the ascension of Claudius Italians were only a bare majority (some 60%) and that the Flavians saw a massive increase in provincial enlistment (such that by the end of Domitian's reign only 20% of the legions were recruited from Italy) such that by Trajan's time only 1% of the legions were Italian in origin is still usually taken as the standard view.