Im sure this has been asked before but is there a way of specifying how many instances of a certain plugin I can add? Thinking of adding vsts such as Kontakt and Omnisphere multiple times as these are 2 of my go to plugins.
 
One way around this would of course to save a template I guess...

As many times as your system will allow until it taxes the audio playback.  Kontakt can be set up in multi-timbral mode, where one instance can be spread over several MIDI channels.  Keep an eye on your performance meter in the control bar to let you know how far you can go.

There is no option to always add multiple instances of a synth at a time. There is an option on the Add Track menu to add multiple instances of a synth on demand. It is the Track(s) entry pointed to by M in this image from http://www.cakewalk.com/Documentation?product=SONAR&language=3&help=Arranging.04.html

Does this take more computing power vs having one multi of Kontakt and using differing midi channels?
 
thx,
      G

As a general rule, a single instance of Kontakt with multiple instruments will be more efficient than many Kontakt instances hosting a single instrument each.
 
However, you may get better core load balancing by using multiple instances. While Kontakt does support multi-threading, NI itself has historically suggested not using that feature because it can cause dropouts, and for years it used to be turned off by default. That position may have changed along the way, as evidenced by Kontakt defaulting to using all cores today. I haven't done any recent experiments to see if that makes a big difference, but in K3 and K4 it didn't.
 
If your machine is running near its limits, individual instruments also give you the option of independently freezing them to free up resources. When all instruments are hosted within a single instance of Kontakt, you can only freeze them as a group. 
 
My own method is to use a combination of single- and multiple-instances. A bass might get its own Kontakt instance, since I'll likely finish it up early in the process and freeze it. A string ensemble or percussion group made up of many instruments will usually share a single Kontakt instance, since they're really being treated as a unit.