I have a set of Wawel Porcelain from Poland that belonged to my great grandmother. It is marked with the W and Made in Poland. However, each piece is numbered separately. Can someone tell me more about this?

Manufacturer was the Polish Fabryka Porcelany Krzysztof, post-WW2 successor of the German Krister Porzellan-Manufaktur AG. As such, your item was created sometime after 1952.

The single number additions are irrelevant as they are merely internal production and control ID codes. A different matter are the so-called 'dating codes' that appear on and off on some items, e.g. "61121-1" which stand for year 19(61), month (12) and week (1) and show the internally keyed ID of the person who did the quality check, internal "-1".

Note that this info is only true for the 'Fabryka Porcelany Krzysztof' between 1952 and 1995 and is no general rule of thumb that can be applied to other manufacturers.