Thank you for explaining. In Russian, nouns after 2,3, and 4 take genitive singular form, and nouns after 5 or above take genitive pleural form. I wonder if Polish is the same. I wish Polish Duo has more official teaching comments/explanations in each topic as in other languages.
It is not the same as Russian, but I guess the line between different cases is the same (2,3,4 vs 5,6,7,8,9)
Contributors are working on the Tips and Notes but it may take a while before anything new is added.
If you need to learn more about numerals I think this can help: