The peak capacity of a CCD pixel is typically 200,000 electrons, and CCDs are demonstrably linear devices up to within a factor of two of this limit. When the limit is exceeded, the arrival of more photons continues to create further photoelectrons, which then spread out along the column of the CCD.
In addition to saturating the chip, it is possible to saturate the analogue to digital converter; in fact, for most of our systems this will happen first.
Together with readout noise, the saturation level defines the dynamic range of a CCD, crudely, as (saturation level)/(readout noise).
Where the readout speed can be varied, the ``gain'' of the CCD is higher (more electrons/ADU) at the faster readout speeds, and, as this is a stronger effect than the change of readout noise with speed, the dynamic range is increased at the faster speeds.