Slugbaby
Along with the good suggestions already mentioned, add a slight pitch shift to some of the tracks.

 
This. 
 
Using the same sound won't give the effext.
 
The sound you want is generated by each instruments not only being at different distances ( time) but the variations in timber and pitch too. 
 
I bet someone has a sample of a pipe band percussion. 
The drums are also very different than what you'll find in a drum kit. 

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.

one old trick not mentioned yet I dont think,  is to play a doubled part a 1/2 step higher...then after it is rendered to audio xpose it back down a 1/2 step.  then vise versa.  Multiple tracks using neighboring samples.

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
 

I have found this to work well. https://www.meldaproducti..om/MMultiBandHarmonizer

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