how many game I need to do for one plat ?

i have been play 7 series game but still fail to get plat i dont know what i can do what i can say , every time there are some unrank players in my prom game and some people intended to feed or not even try to win, whats going on with this league?? this is gold 1 game why i can met some people with 1400~1500 MMR ???