I have defined a series of custom post types (namely “PortfolioPage” and “PortfolioGallery”).
Each of them uses a custom taxonomy called “Artist”.
Is there any way to limit the end-user to enter AT MOST ONE artist per PortfolioPage / PortfolioGallery post?
You could cause the custom taxonomy to use radio buttons rather than checkboxes, have a look here: https://wordpress.stackexchange.com/questions/7456/altering-the-appearance-of-custom-taxonomy-inputs