Well, I tend to think someone with more experience in orchestral conducting would be better suited to answer this. (Petr? Andy Brick?) I would also say it depends on how you're using the synths: a synthesized violin section doubled by 2 or 3 real violins on the same part can sound an awful lot like a whole section of real violins, AFAIK. Beyond about 3 or 4 violins, increasing the number of instruments on the same part does more toward increasing the "sound mass" than changing the timbre, and a synth could do the same thing in terms of mass, while the real violins add that timbre that only a real violin can so far give. I think the numbers would be similar for violas and celli; I'm not sure about basses.