Should You Stick With One VSTi, or Lots?

This is a good discussion for many reasons, your instruments and how you use them truly define your sound. The VSTi synth sounds are by far the most recognisable aspect of many artists and for good reason, they know and stick to the sounds they like.
I would say the most important consideration in this discussion is to look at what your main Synth can do. Massive for example is an extremely powerful synth capable of little short of everything. If you were lucky enough to own a copy of that, you could be forgiven for not using any other synth.

You cannot go wrong with a fully capable synth.

If you use a simple VSTi, perhaps because you aren't sure what half the settings do, I recommend downloading many more. The more synths you play with, the more you learn about synthesis. Yes they will all have different GUIs but you will realise very quickly that the principles are very much the same.

If you want to stick with using one synth, I suggest a synth that is capable of at least two types of synthesis, Subtractive/Additive, and FM. If it can do more, brilliant. The reason for this is because the amount of sounds you can make multiplies with more than one type of synthesis and, if it's in the same instrument, you already know your way around shaping and playing with the sound. 

In the end, I always like learning new synths, even just the difference in how a synth looks can inspire different sounds to be made, simply as the workflow is slightly different. It's great to try and recreate presets in one synth into another and failing at these excersises are often what makes you discover new sounds.