Last I knew, there were fewer than 150 in the series, so that means that if you've been a JW for more than 3 years, you've probably heard them all (or heard certain ones multiple times). If you were a JW for almost 30 years, like I was, then you've practically got them all memorized.
I don't think that it always used to be that way but I believe in the effort to make things more uniform, the WT developed the select number of outlines that were only to be used for Sunday talks. I think that before the uniform outlines, local elders could be more liberal in their talk outlines. The problem was that sometimes elders would be very liberal and go off into some weird tangents, thus the need for preset outlines. Of course, elders will say that they are only outlines, and they can develop each outline as they see fit. The problem is that they are all looking at the same info, no real outside sources, so they all begin to sound the same. Plus, once an elder has a outline done, odds are that he's not going to redo it, thus, you''ll hear his same outline over and over and over, maybe with a new illustration thrown in, then over and over again. Most elders only have about 5 to 15 outlines out of the 150 done, but I did know this super dub in my hall that had them all done, go figure.