So, for example Planescape: Torment would have much less encounters if it was a PnP campaign? How much less?
Hard to say, it depends on too many factors (in general the adventure module, your DM, the PnP system you are playing, etc.)
And then the question is also, what number are we looking at? Fraction of time spend on combat? Encounters per hour? Absolute number of encounters?
I had sessions lasting several hours that consisted of nothing more than one big combat. On the other hand you can have role-playing-heavy modules where you have only maybe 1 or 2 combat encounters over several sessions.
But staying with PST and AD&D, it would still depend quite a bit on what parts a DM wants to emphasize.
Since the strong part is the story and the writing, one could well play it with significantly less combat, imho.
Also one should keep in mind that in a group with several people even one combat round can take a few minutes, so unless you want the campaign to slow down to a crawl, you'll probably want to remove a good part of the trash encounters - do you need 4 enemies behind every corner, even when they are weak? I'd rather try to "set the mood" - a small dungeon might contain a few (say 1-4) separate encounters with fitting enemies, for a wilderness area one encounter can be enough already and bigger fights I'd do only at important parts of the story anyway.
Of course a larger multi-level dungeon can have more encounters, but that might be enough material for an entire module already.
Which is another thing to consider: A cRPG can contain quite a lot material you can consume faster than in PnP, even without trashmobs.