how to make one instrument sound like many

IMO if you want to make it convincing you need to play the parts at least a few times before copy-pasting and nudging or using doubler-VSTs or whatever. I don't know if there are any one-VST-solutions for that purpose. My guess is that such exist.
Of course, if you have, say, a MIDI drum track, you can copy it and slightly change the timings and proceed from that point. Creating a realistic sounding ambient (like a marching band on a market square) can be a bit tricky.

I agree with Kalle, playing the part multiple times gives the best sound. Using techniques like cloning a track and nudging it slightly ahead or behind to make it sound like another part does create what you're after but often lacks what makes multiple instruments sound like, well, multiple instruments.
There are subtle, and not so subtle, differences between parts played over again as opposed to cloned tracks. In some situations it probably doesn't matter if the track is cloned, but if you want a bagpipe band then there is no substitute for a lot of bagpipes.
Even just three or four parts that you then clone and alter to make them sound different, is better than just a single track. IMHO.

Playing multiple times is a good tactic, you'll get slightly different timing for each one - definitely don't overquantize and repair manual screw ups if they are too bad. Assuming you're going for an "acoustic" sound, judicious panning and use of reverb also helps. Try to give each part a unique place in space - left and right but also forward and back (with reverb). Use different patches if you have them. If you don't have enough subtly different sounds, use a little EQ to make them sound different. Remember that brighter also sounds closer. You can also use Melodyne to subtly detune some sounds, and even change the formant (overtones) of them.
 
All this is a lot of work to get to sound convincing, but you can get there. Try to put the more "processed" sounds near the back and take a few minutes to plan it out.

Melodyne Studio 4 has “Polyphonic Spectral Shaping", and in the following video starting at around 5 minutes, it demonstrates taking the spectrum of a Clarinet, and copying it to a Saxophone, giving it the characteristics of the Clarinet.
 
https://www.youtube.com/watch?v=_IF_wF0aGJo

Maybe this?
http://www.quikquak.com/prod_crowdchamber.html

If the instruments are midi, you could copy the tracks and quantize them all slightly differently, pitch shift slightly, use slightly different sounds. But like others have said, the best way is to layer up multiple takes to get the crowd effect.

cmyst
Hi all
Is there a way to make one instrument sound like many - ie bagpipe band? multiple marching snare drums? (for example)
I am trying to avoid playing the same thing multiple times
Can't seem to find any loops
Kind regards
 

Yeah, there are several ways.  The method I use is as follows:
Let's say you have 1 trumpet sample-set.   But you'd like the sound of three playing the same thing.
 
1.  Use 3 separate midi tracks all playing the same thing with the same instrument.
2.  Detune two of the tracks, one sharp by 10 cents or so, one flat by the same amount.
3.  Start one of the 3 tracks a little late; move the notes forward by 10 or 15 ticks or so and make one of the 'tracks early by moving notes the same amount before the beat.
 
You now have simulated a chorus effect, which is almost as good as having a sample-set of three trumpets playing.
 
Jerry
www.jerrygerber.com
 

Wow!! Thanks so much guys. Didn't expect so many interesting and varied replies.
I agree that playing the parts multiple times will give the most realistic effect but I will try all of your suggestions and see how they sit.
 
Greatly appreciate the responses and suggestions.
Kind regards