Well, speaker placement is kind of subjective. From front to back:
There is a place smack in the middle of the top of the dashboard for a factory speaker, or if you check with the vendors, an aftermarket dual cone solution (which sounds WAY better, but still not very 'stereophonic').
Depending on the size, you could possibly squeeze them into the kick panels with some Q-logic-type aftermarket kick panels, expect them to interfere with the e-brake pedal though. Check Greg's (Austin Vert) thread about how he solved that issue (basically modified the e-brake pedal to work).
If your door panels have the 'grilles,' you could put some in the lower front corner of each door (where the factory 'deluxe' speakers came, if I remember correctly). Depending on the size, some trimming of the door metal and/or door panel might be required.
A popular place (although, unsightly and very damaging to the interior) is the rear quarter panel, just below the quarter window. Again, making holes in the metal as well as the rare plastic trim panel. I wouldn't recommend doing this, but if your panels are already 'pre-cut' by the previous owners, it might not be a bad way to spare having to buy new panels.
Then we have the rear deck. On fixed rear seat fastback models, there are all sorts of places for speakers to go there. Pulling out the package tray trim panel will expose lots of different speaker configuration possibilities. I believe 5x7s and/or 4x10s will drop right into the factory holes. With some 1" spacers (on top of the trim panel) you could fit some 6x9s without having to trim anything (except drilling some mounting holes, of course... unless you can get creative and somehow use the factory screw holes to mount the spacers, then simply mount the speakers to the spacers that is).
As well, that space immediately under the package tray is just screaming for a custom fit box with a couple (or three) of subs. Build the box with the speaker cones firing into the back of the rear seat - it will offer the best protection for the cones. Built the box correctly, and you can even have room for mounting the amp(s) and other electronics you might need to support the system.
As far as fold-down seats, 'verts, and coupes, I think someone else will have to chime in, as I'm not familiar with those rear seat/package tray areas.
How many speakers? Well, again - subjective. I'm thinking a pair in the center dash, 5.5"s in the lower door panels, 6x9"s in the package tray, and a pair of 10"-12" subs (w/custom box) for mine with a single amp to drive the subs. Depending on how things go, I might put some 5.5"s in the quarter trim panels (since there are already holes there) - I'll know more if I can restore the panels or not.
On a related note: I have a similar set-up in my Ram (minus the dash speakers): JVC CD/MP3 head unit, Infinity 5.5" component speakers w/separate tweeters in the doors (to replace the factory 6x9s), Infinity 5.5" dual cones in the 'B' pillars, and an MTX Thunderbox w/12" MTX subs. The head unit drives the Infinitys, and the Thunderbox is driven by an MTX Sledgehammer 1000 amp. The system cranks and is absolutely crystal clear - it runs up to 50 on the volume, and 40 is about all I care to handle (which is freaken loud... trust me) - I typically listen to it in the 28-32 range during normal driving.
