A fuller answer:
Will Planner work appropriately for having a large number (say 8) goal seeking characters moving around or would that bog it down too much?
That should be fine. It's the number of planning rules that will determine speed. (Any rule, multiply the time it takes by the number of people you have.) So if your model world doesn't use, say, locked doors as obstacles, then don't add the rules for locking / unlocking doors.
Also, you can list what order you want your planning rules in. Put the commonest early in the rulebook and the rarely-used last. There might be some constraints on ordering to prevent problems that I get into below.
Do I need to create a separate table of plans for each of the NPCs or do they share the same one?
IIRC, I believe they share the same one. IIRC.
In Planner, if the primary goal is met early for one NPC, but then through other NPC actions becomes unmet, is there anything special I need to do to get the original NPC moving again or will it happen automagically?
Automagic, but with a caveat. Planner NPCs have no memory, so yes, if an NPC undoes another's task, the first one will ensure it is done again. The problem is, NPCs do their task list in order, rather than seeing which one of a group of tasks would be efficiently accomplished if it is done next. So if an NPC undoes another's task, the affected NPC will likely drop what he's doing to go re-do the earlier task. That will likely look unnatural. Example: if the middle task is to Open the Iron Gate, the NPC will do so, passing through it and doing whatever needs be done on the other side. But if a later NPC closes the Gate after walking through -- even if it was simply because he's tidy -- the first NPC will immediately go back and open the gate, then return to what he was doing, even though whether the gate is open or closed is moot once you're on the correct side of it!
Obviously, the AI programmer should state the first NPC's goal as "be in place X" rather than "open and go through door Y" to prevent those kinds of snafus; your planning rules will instruct him to do whatever needs be done for conquering obstacles. You'll also need to write some rules for the purpose of debugging what all NPCs are doing, and look carefully for inefficiencies of that sort. It's not enough to know what
an NPC is doing, which will be frequently correct, but you must also keep in mind why
he is doing it, because that's where many AI errors occur.