Entirely depends on their level of commitment. If they're just intended to compete for 'RGT' honours in the overall classification, then I can't see that they'd sell that many. If they put some money into promoting a 'Cayman Cup' structure, similar to the circuit Carrera Cup model, taking away the focus from the overall classification, at the right price with a well made car, maybe it's possible. Didn't Porsche say they'd be looking to sell around 100 to make the project feasible? It's a big ask, but I can imagine national 'Cayman Cup''s within the main national Championships in France, Italy, Spain, Belgium and maybe Germany being able to get ten entrants each. If that is possible you've already sold 50% of your target. Once you take into account other, most probably much weaker, Cayman Cup's in other countries, random Gentlemen driver sales and collectors, replacement cars over time, maybe you can sell another 50. That's without taking into account the possibility of an international Cayman 'Supercup'.
As for not fighting for overall honours being a problem, I can see the logic of not wanting to be beaten by a bunch of Skoda's, but in endurance racing Porsches often haven't been fighting for overall honours. Separately the Supercup and Carrera Cups are just sideshows to the main event, be it F1, DTM or BTCC ect... GT4, in which the Cayman competes, is a good few rungs down the sports car ladder.