One can estimate a reasonable plate number for any column as:

                         N ≈ 300 L/dp

L is the column length (in millimeters) and dp is the particle diameter (in micrometers).

For example for 250 mm x 4.6 mm column packed with 5 micron particles, the predicted value is

                                       N ≈ 15,000   

If the measured value of N is about 20% of this predicted value, the column is in good conditions.