I have several VST instruments like NI Kontakt or UVI Falcon, which can load several sounds on different MIDI channels etc.
 
Now if I want to use several Kontakt instruments, do I create many "Instrument Tracks"? Or do you create one and then some MIDI channels sending to that instrument? I'm confused what the correct mehtod is, so CPU and RAM get conserved properly with such instruments and DAWs.

I have the same question, confused about Garritan Instant orchestra

Hi Phillipe,
With both Kontakt and UVI, you can create one instance of each and then add multiple instruments.
From there, you can add audio tracks in Sonar and assign them to the channels you assign your instruments to.
For instance...
In Kontakt, let's say you add a piano and a horn instrument.
You can approach this a couple of different ways - 1) you can have them both play on the same audio track or 2) you can assign them (first in Kontakt) to a new/different output and then assign them (in Sonar) to a corresponding audio track.  This does keep things cleaner.
 
Just a note - with the UVI module, I noticed before I would get static if I had multiple instruments running in one instance.  If you're using the more current  version of Sonar Platinum, it's probably nothing to worry about.  Just mentioning it because it happened before.  I haven't tried it on UVI since (haven't needed to anyways)..
 

Most people do not use instrument tracks for multi-timbral synths because they do not offer as much versatility as MIDI-Audio track pairs. But they'll work if that's what you're more comfortable with, just with fewer options.
 
You're usually going to set up the sampler one of two ways: separate outputs for each instrument routed to separate audio tracks, OR use one stereo pair of outputs into a single stereo track and do all the mixing within the sampler itself. The former method is best for dissimilar instruments, e.g. a percussion instrument + a piano + a string section, or for a mix of mono and stereo instruments. For like instruments, e.g. string ensembles with one instrument each for violins, violas, cellos and basses, it's more convenient to mix them within the sampler and then treat them all as one instrument for the main mix.
 
Usually, each instrument will get its own MIDI channel, so if you stack 16 instruments in one sampler instance you'll necessarily have 16 MIDI tracks. Sometimes two instrument might share a MIDI channel, such as when layering kicks, generic string patches or synth leads. Sometimes you'll want to clone a linked MIDI track to drive two instruments from the same MIDI data (perhaps transposed). "Instrument Tracks" don't give you these options.
 
I do not have a single method, and don't believe there is any one "proper" way to do it. It comes down to whatever works for you. I will, however, note that more often than not, I'll use a separate MIDI/audio track pair for each instrument; 16 instruments = 32 tracks. I like to keep all my options open.

I have found that running a separate instance of Kontakt for each instrument better suits my workflow and offers a few advantages. It provides separate midi and audio tracks for each sound without any additional patching, makes very little difference to resource usage compared to multiple instruments in a single instance of Kontakt, enables me to easily freeze or bounce individual sounds and allows for the creation of individual track templates for favourite sounds to load them singly as required.
 
As others have said, there really is no 'correct' way, just the method that suits you best.

Kontakt is multi-core aware and will use them if you enable this in settings.  Unfortunately, Falcon is NOT... very frustrating but apparently this is not an easy change for uvi... it will always hog the main core...
 
As to Omnisphere, which can use every bit of your cpu rather quickly, see this