For the number of nids, more is more. There was an armies on parade entry (i think) that had hordes of nids swarming all over the place. if you go by fluff one SM should be able to handle a fairly large number of these.
Scale wise for the cliff, look to nature to determine this and then work backwards. We know a SM is 7-8 feet tall (give or take), so find some cliffs with people beneath them to give a bit of scale perspective. a 30 foot cliff versus a 7 foot SM is about 4:1, so a 28mm SM x 4 would be 108mm ish cliff face. now, 30 feet isn't very high if you are trying to show the hopelessness of this situation and the desperate nature of the fight. so scale up to say a 100 foot cliff, would be a 14:1 ratio or 14 SM high, making scale about 40cm give or take. That would be very imposing and but may be difficult to make stand up.
However, some of the train diorama people using foam blocks and cut these and shape them to make their high hills and cliffs, so might pay to have a look at how they do it.
Hope that helps a little.
"Even the broken, break" - Haemonculus Mykalei Stanne, Coven of Dark Tranquility