i am not a teacher at all, but i will try to clarify it. When we just count , without an object, we use ordinal numbers. For example, one,two, three and etc.(один,два,три). But, when we have a coherent object, i mean , when we need to count something that is definite, we use genitive. ( одна кошка, две кошки, три кошки (one cat, two cat, three cat)and it is important to notice a group of a noun ( i mean a gender) because in Russian a noun" cat" is female and we use "две" , but if we have a masculine gender like an object ( like a glass of water) we say один стакан воды, два стакана воды, три стакана воды) . Thus, if you just count, ( one two three) without an object and when you count a noun that is in a masculine gender, you should use ordinal numbers ( один два три) . and when you need to count a noun in feminine gender , you have to say" две" сестры ( two sisters ).

Hi, Bruno! It has to do with grammatical gender. Книга in russian is female. Compare: Книга (f) - Одна книга, две книги. Стул (m) - Один стул, два стула (One сhair, two chairs)

So it would be два папы? - YES

So it would be два папы?

Beware of nouns that are feminine grammatically (папа, дядя, Ваня) but are masculine in nature because they refer to male persons. You will have to use "два" with those, not "две"... Tricky, I know... Goodbluck! )