In this case the limit is how high poly a model you can realistically UV and texture. A multi million poly raw zbrush Dynamesh model won't be something you can load into Substance Painter or Mari. You'll need to create a lower poly model that you can texture and then either, use displacement, or reproject details onto it…
For film the limit is what your rigger can handle. You cant animate a multy million poly model. You need a subD model with displacement or a normalmap on top. Just use as many poly to get the subd shapes and blendshapes you need. I would aim below 1mio poly.
Cool. Thanks for the advice. What I'm planning is actually a character bust for the sole purpose of practising facial likenesses, real-time hair, and hard-surface modelling. So no animation/rigging required. :) I was advised to incorporate a bit of a film/Virtual Production workflow, so I'll be able to use UDIMs and have…