What percent of Ukrainians think of themselves as Russians?
This we have to ask the people. Luckily, in 2001 there was a census and 17% of Ukrainians said they identify as Russians: this is the answer.
does Russia have an justification for taking the territory of another country?
This is a relative question. Russians would say that yes, they have citizens in this other country that they need to protect, but the international community sees is as a violation of independence of Ukraine and sees it as Russia trying to gain ressources and Territory