So far, it's not possible (without dynamic patching), but there's a WIP PR which I should probably reactivate: https://github.com/pure-data/pure-data/pull/1069
Yes, please reactivate! DSP switching itself is not so much of an issue as you say yourself in the PR, but a settabe number of instances would be fantastique!
In the meantime, just set the maximum number of instances you would possibly need and then deactivate the ones you don't need (e.g. every instance with a number >= N).
Not a bad idea, thanks Christof!
P