The 32-year-old netted 16 times in the Champions League in 2015, and has equalled that number already in the past 11 months, but can become the first player to net 17 goals if he can score in either of Madrid's final two group games.
Zinedine Zidane's side face APOEL on Tuesday, November 21 and finish their Group H campaign at home to Dortmund on December 6.
Ronaldo has had the best three years in the Champions League from a goalscoring perspective, with 16 in 2015 and 2017, and 15 goals in 2013. The next best is Lionel Messi, who scored 13 times for Barcelona in both 2012 and 2016 - something Ronaldo also did in 2012 and 2014.
The Portugal international also has the best goalscoring record in a single Champions League season, with 17 goals in 2013-14. This season, he is also the leader in the top scorer race with six goals, ahead of Tottenham's Harry Kane, who has scored five times so far.