Why Rome had so many First Legions

Were Roman legion numbers or names reused?

Not as one simple sequence; several provincial authorities could each raise a first legion, so the number I appeared repeatedly.

Familiar legion numbers and names were not reused in the straightforward sense of replacing one legion with another bearing its identity. The many First Legions instead arose because senatorial or provincial authorities could raise a formation within their own sphere. Such a force could be the first legion of its province even while other Legio I formations existed elsewhere. Roman numbering therefore reflected several origins and chains of authority rather than one tidy empire-wide sequence.