A poll conducted in 2014 by LifeWay Research for Ligonier Ministries might help answer this question. You can read the article here, but according to them...
Most American evangelicals hold views condemned as heretical by some of the most important councils of the early church.
...nearly a quarter (22%) said God the Father is more divine than Jesus, and 9 percent weren’t sure. Further, 16 percent say Jesus was the first creature created by God, while 11 percent were unsure.
More than half (51%) said the Holy Spirit is a force, not a personal being. Seven percent weren’t sure, while only 42 percent affirmed that the Spirit is a person.
These poll results are specifically for evangelicals and American Christians, and it seems to be the only poll of its kind. I've personally heard several heretical analogies used to explain the trinity, such as comparing it to H2O (modalism) or an egg (tritheism). This article does a good job of explaining all the bad analogies that get passed around, by possibly much more than 50% of trinitarians. The article concludes with...
One more thing. I often tell my students that if they say, “I get it!” or “Now I understand!” that they are more than likely celebrating the fact that they are a heretic! When you understand the biblical principles and let the tensions remain without rebuttal, then you are orthodox. When you solve the tension, you have most certainly entered into one of the errors that we seek to avoid.
So 98% of Christains belong to a trinitarian denomination, and if any of them think they understand the trinity, they're a heretic.