How many samples are needed for a calibration?

One of the questions normally asked is: how many samples are needed for a calibration?, for how long I have to add samples to a calibration.

Of course what is necessary is calibration data from different years. At the beginning we can have a nice SEC but not so nice SECV or SEP, but as soon as we have more data from next years we will see how the SEC is increasing and the SECV and SEP are decreasing and are becoming closer to the SEC and the continue to become similar, but not bellow.

The idea is to continue adding samples and variability while SECV is significantly different than the SEC and while the SEP is significantly different than the SECV.