The answer for me is zero. In fact, I don't think I remember any sermons from the first twenty years of my life. Now that I am nearing forty, I can say that the actual number of sermons I remember from my entire life probably numbers fewer than twenty.
In addition, I'll sheepishly admit that I don't remember most of the sermons I have preached over the past few years.
Why is this? Why is it difficult to remember sermons? The reason is that God did not make our brains primarily for one-way communication. I'm not suggesting that we can't learn this way, but rather that the much better way to learn is through dialog/conversation. I think we would all agree that we retain much more information through engaging, back-and-forth conversation.
How then does this go along with the gift of teaching mentioned in scripture? As a church comes together, there may be a few people gifted in teaching who do most of it (although I hope not all of it). When the teaching takes place, there is no reason for it to be in the monologue/lecture format. Since everyone learns better in multiple-direction conversation, the teachers should ask questions and expect answers. Others ought to be free to add their comments. It should be interactive.
Interestingly, this is the type of communication that does happen in Sunday School classes and small groups. So why do we do this in smaller gatherings, but when it comes to large church meetings, the communication (at least for the sermon) ends up being in one direction? Part of the issue is pragmatics, such as how 500 people can all be involved in a group conversation. That, in turn, says something about the importance of keeping churches relatively small in size (but that is another issue).
Something else interesting about the years past: while I don't remember sermons, I do remember adults who took time to care about me. Some of these adults didn't even spend a great deal of time with me, but I remember when they did. For whatever reason, this really sticks with me.
As I grew up, I'm certain that I spent far more time listening to sermons than I did speaking to adults outside my family who took interest in me. Despite this, it is the adult interaction that made the much larger impact.
What does all this say? I'm not sure. Here are a few things we should ponder:
-Sermons, if they are going to happen, should at least have an interactive component.
-Teaching is much more effective if the conversation is multi-directional.
-As adults, we can have a hugely positive impact on young people.
-As we think about importance of activity, spending time with younger people is equally (if not more) important than listening to sermons.