Placement matches details

i was wondering how placements matches work, im gold 2 trying to be gold 1 but want to get plat next season at least, from experience i got rated higher then i finished the season and didn't do good, were as my friends did better and won more games then me and got placed lower, i finished s3 s5 and got placed at 4/10 in s2, so i was just wondering how many games it would take me to get at least back to my original rating