You can really get a fat sound regardless of how many voices you use. Obviously you want a couple just to have it be a little more spacial but, the more voices you have, the nastier phasing you're gunna get. For any pluck I make in Sylenth1, I usually use 3 oscillators for the saw waves, each at 2-4 voices, and I have one tuned an octave down, one normal, and the last an octave up. Adjust filter envelope as necessary, and just find little tricks here and there to make it sound bigger. Reverb plays a very big part, and I usually stray away from the built in reverb on the Sylenth, as it sounds very metal-y and doesn't compliment plucks very well (at least I don't think). There are tons of easy tricks to make it sound bigger and have a louder attack that can be easily done. On the fourth oscillator that I'm not using, sometimes I'll throw in a noise wave (at extremely low volume, like 2%) but it gives it a nice sharp tone in the beginning. Also, compression with a healthy attack on it can make the beginning of the sound pop a LOT if done right. A lot of times I'll hear people make the decay on the filter envelope way too short, so you barely get a sound other than an initial 'pop' of tone, as they are trying to recreate the loud transient that some of the pros have, but thats definitely not the way to do it. Reverb & filter envelopes are your best bet, and also the comment above about saturation is a very good idea as well, and could really give your sound some low-mid meat, which is always good! Hope all of this helps