I don't think I could make a list of how many different titles I'm following at the moment without looking at a list. It has to be around 25 or 30 currently, though several of those aren't finished but may never get a continuation.
And that's only here, that's not including physical books. Which is where I'm actually very far behind...
Thankfully, I am a very fast reader. I also have read at least some before I go to sleep every night for as far back as my memory goes. And even as a kid, it made perfect sense to me to be reading 4 or 5 different books at once... one I would take to school, one in my mom's car, one at my bed, one in the living room, etc. So I am rather uniquely well suited for J-Novel's format.
As much as I read, I haven't dropped too many series. I have a good feel for what titles I'm probably not going to enjoy so 1 part is plenty to make the call. The ones I have dropped were all because the protagonist bored me or because I simply couldn't build any sense of being invested in what was going on.