The ADC is only capable of 93db SNR, line 3 in the features in page 1.
And this is only with the pre-amp gain set to 0db or less with the minimum amount of distortion while sampling a source signal at ~ -6db, or half amplitude. Yes, sampling the full amplitude reveals additional distortion as seen on figure 25 on page 43.
Only use this device for consumer audio, do not expect studio grade performance even if the data bus has more than 16 bits.
As for the true bits of the ADC internally itself, it is not specified.