If I see a sequel and haven't read the first one, I don't usually jump into just reading the sequel. I'm most likely not to read either, but if the summary of the sequel is compelling enough, I have been interested enough to end up reading both.
The more sequels it has, the less likely I am to continue, unless the writing is good and I'm enjoying it. I'm trying to remember how many stories I've read that have sequels to them that I've read too, but I can't quite remember (meaning, there's a lot of them). For the most part, it's just one sequel. I can think of a current story I'm reading that's on the third story in the series, and it's been really well-written, so it's got my interest still. There was another series that was pretty decently written, but after the second story of four, I haven't found the motivation to read the remaining two yet (if ever), probably because of how far it's gone away from canon.
The most number of stories in a series of fanfics that I can remember reading was six or seven. They were pretty short and each centered around a different character. It started with one character finding out the main character's big secret in the first story, and pretty much each sequel was a kind of domino effect, as each focus-character of each sequel found out the secret in turn (whether because one of the previous characters to find out accidentally let it slip or they discovered it on their own because the main character's behavior had changed or whatever).
There is one ongoing story that I can think of that is a sequel (and most likely to be the end of the story), and it is doing exceptionally well. If you were to search the category it's in on ff.net and sort by fave/follow/review etc, it's within the top stories listed. Taking the following filters, this is how it measures up to the rest of the fanfics in the category (out of 17k fanfics):
Sort by Review, Rating All - Story 1 is 4th and its Sequel is 15th in the entire category
Sort by Favorites, Rating All - Story 1 is 3rd and its Sequel is 14th in the entire category
Sort by Follows, Rating All - Story 1 is 3rd and its Sequel is 2nd in the entire category
So, out of 17,000 stories, it's not that surprising to see it at the top. It's excellently written, you can follow the author's progress on tumblr (she posts sneak previews of the next chapter sometimes), there's fanart out there for the story, it's updated consistently (once a month), and it's one of the types of storylines that fans of the series love to read/write about (it's one of the best executions of the trope). Breaking it into two separate stories makes a lot of sense, because the first one is very much about one thing, while the second is a combination of aftermath from the first and a new storyline that's integrated in there (not to mention that the characters involved are different... the first one was limited to focus on three characters, the second one includes many more that were unable to be in the first one and can now react to things that happened in the first one by being present).
As for writing sequels myself, I've only got one that I'm planning, and it's probably going to stay that way. Both stories have really tight storylines, but it makes a lot of sense to split it into two, since one is very much focused on one thing while the other is focused on a different thing (and different characters are more central in one than the other). It works well as two stories. I haven't started posting either story yet. I'm planning on using some things I've learned from observation of other stories (and from marketing) to help boost the readership of these stories.
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