For correlation studies I only do 5-10 samples. I make sure to cover the range of values for each. For example the samples I run for our urine analyzers I try to have samples that are positive for glucose and bilirubin. I also make sure that the Specific Gravity covers a wide range as well as the pH. This helps me to be sure the refractometer and manual pH results also are covering the low, normal and high ranges. Our Joint Commission inspection this year was fine with that.
Lisa