The amount of polyphony that Notion for iOS is able to handle is related to the size of the CPU in your iOS device. It is also dependent upon the notation that you have in your score. The warning is telling you that once you go avoe 14 or 16 instruments, you could possibly deal with audio distortion and artifacts. If you have 32 instruments in a score and only 10 instruments are playing back at a time, then you may not experience a problem at all. Another factor here would be sustained notes. If you were to only have one instrument in your score, say a piano, and you added a pedal down marking without any pedal off markings, then each note would sustain. Eventually the sustained notes would be compounded over time and you could run into a polyphony issue with only one instrument.
To sum things up, the more powerful a processor that you have in your iOS device, the more polyphony that you will be able to support. When measuring the number of instruments that you can have in a score, this can vary from score to score, due to the number of instruments that are playing at a given time as well as any sustaining effects that might be used.