The only logical point at which a follow-on process becomes too much, is when the code becomes a duplicate of previously posted code.
Follow-On questions are important on this site, they add a lot of value as they often lead users through a progression of steps resulting in great code.
My experience is that, when a particular user takes their code though a series of follow-on questions, I start to lose interest if there's nothing new to say, so I simply look at other questions, and then, later, see that there's been a new answer, with a different perspective, or insight.
I don't see there being a limit to having "too many follow-on" questions, and, having a 'dangling', unanswered, 'last item' in the chain, is not a long-term problem. Someone will come along and answer it, sometime.
If you are concerned about this as a problem, it may be more because some people who post lots of follow-on questions are also the people who don't apply previous suggestions to their code, so are not improving their code in certain ways between questions.
I find people like that to be frustrating, but there's nothing technically wrong with people who don't follow the advice they are given... I just start ignoring them... if they ignore my suggestions.... and move on.