If you want to do 16 cellos, you can set the polyphony to 16. (which may be the default anyway?)
It would be wrong to combine all strings into a single preset, since each has it’s own characteristic sound. Still, there are generic ensemble programs in HSO. Still, you could create a HALion Sonic track, then load various HSO instruments into the 16 slots, set them to trigger on the same MIDI channel, and use the HALion Mixer to balance them to taste.