Every pair of outputs being used will "instantiate" a different Mixer plug-in, though they can share the DSP with other Mixer plug-ins. For instance, routing 10 tracks to outputs 1-2 and another 10 tracks to outputs 3-4 will need 2 different Mixer plug-ins - one would be a 10x2 mixer for outputs 1-2, and another 10x2 mixer for outputs 3-4. The mixer always grows by using up the maximum DSP for a single output pair, so you always grow up to 59x2 (68x2 on HD) before "splitting" across DSPs. It's possible that the order of adding tracks, etc. can affect when the "splitting" does need to occur, but remember that any individual output pair doesn't "split" until it has more than 59 inputs (or 68 on HD).