"In the room i've two windows." is not very idiomatic English. "In" and then "have" do not flow together like that in English, "have" suggests a possessive pronoun. You could say, "In my room, I have two windows" or "In the room, there are two windows".
I'd say that "I have two windows in my room" is more grammatically correct as the adverbial modifier of place is put in the end.
case | masc. & neut. | feminine
nominative | два | две
genitive | двух | двух
dative | двум | двум
accusative | два, двух | две, двух
instrumental | двумя́ | двумя́
prepositional | о двух | о двух
There are only two numerals having a feminine version: один (одна) and два (две), один also has a neuter (одно):
- один мужчина / одна женщина / одно яблоко
- два человека / две женщины / два яблока
And we don't say "два мужчин" or "два детей", instead we use a collective numeral двое: "двое мужчин"
I believe it's used with feminine nouns and два is used with masculine and neuter nouns.