We can get a rough idea of this by considering how much physical changes have a mental effect. Psychoactive chemicals, brain damage etc. Look at how much ethanol changes the behaviour of a single neuron in a lab dish. How much it changes human behaviour. And that gives a rough indication of how sensitively dependant human behaviour is on the exact behaviour of its constituent neurons.
The question is not just “how much is needed” but also “what’s a reasonable difference between the new digital mind and the biological sustance”.
We can get a rough idea of this by considering how much physical changes have a mental effect. Psychoactive chemicals, brain damage etc. Look at how much ethanol changes the behaviour of a single neuron in a lab dish. How much it changes human behaviour. And that gives a rough indication of how sensitively dependant human behaviour is on the exact behaviour of its constituent neurons.