More signposts and squences.
Question: Would having more than four signposts in your Overall Story Goal or elsewhere be okay?
There could be one more thing that pushes your protagonist to change- more then one impact character, for example.
In addition, if your planning a series, shouldn't you have some idea of where your foreshadows will go?
Dramatica theory observes that, in well constructed stories, the overall throughline moves through four stages because the throughline is an exploration of a particular thematic domain and each domain has four parts or aspects to it. A complete exploration is required for the story to feel complete
For instance, if the overall throughline for a story is in the domain of Situation, the plot will move through an exploration of the Past, Present, Future, and Progression (not necessarily in that order). An example of this would be A Christmas Carol
in which the four stages of the overall plot revolve around the main character, Scrooge, encountering four ghosts: Marley (whose focus is on progression), Christmas Past, Christmas Present, and Christmas Future.
The overall throughline of many Hollywood films and genre stories are explorations of the domain of Action, and move through the four stages within that domain: Learning, Understanding, Doing, and Obtaining (again, not necessarily in that order).
So, the four signposts represent each of the four stages. Sometimes a signpost is just one event. Sometimes it is a sequence of events. And sometimes it can repeat, that is, the Past might be explored through more than one event.
Whichever way you choose to explore the four stages, exploring them one at a time creates the feeling of Acts.
(Note, however, that once you have explored a signpost and moved on to the next, you do not return to the previous one. Scrooge doesn't revisit Christmas Past once he is done with it.)
Of course, there are also three other throughlines with their own sets of signposts, which is why some story models talk about additional stages to
the plot. As far as I know, only Dramatica has delineated the 16 signposts (four per throughline).
Also (just to make things more complicated), some people are more comfortable with a three-act rather than a four-act structure. For them, the acts are the transitions between the signposts. Act 1 goes from Signpost 1 to Signpost 2. Act 2 goes from Signpost 2 to Signpost 3. Act 3 goes from Signpost 3 to Signpost 4.
And there are also writers who use a combination approach involving seven stages (four signposts + 3 transitions).
Regarding your second question (Can there be more than one thing that pushes the main character to change?), generally there is one key issue the main character faces that is crucial to the outcome of the story, and one impact character that forces the main character to reconsider his approach to it.
Of course, all rules have exceptions. Sometimes, you can have a main character with no set way of handling things, and therefore no reason not to accept the impact character's advice in Act 1 - which would mean no inner conflict. So in these cases, writers sometimes provide two impact characters with opposite approaches so that the main character must decide which of the two he will emulate.
A novel can also contain more than one story, each of which creates its own dilemma for the main character, which would also warrant more than one impact character.
For instance, if you are writing a series, you might have a Story Goal for the entire series as well as a separate Story Goal for each book. Each book would present a complete story, but at the same time be only one Act in the series.
Planning the series story ahead of time allows you to foreshadow events that will occur in later books. And, yes, I would recommend that you have at least a rough outline for the whole series so that you can set the reader up for what's coming.