[BroJoeK]

SunkenCiv: "The Roman army after the reforms of Augustus consisted of about 28 legions (140,000 soldiers) of regulars..."

Check out this link.

Note that in the case of the Sixth, there were more than one, including the Sixth Ferrata ("She Wolf") in Galilee, the Sixth Hispania which fought in Bulgaria and the Sixth Victrix in York, which built Hadrian's Wall.
Similar alternates for other legions.

The explanation is that different emperors began numbering their own new legions starting at I.

[SunkenCiv]

Thanks BJK.