Yet this would then increase spoilage significantly would it not?
It could be an additional multiple choice field, with N+1 options. Both would need to be filled in for the form to be valid.
I'm not sure I understand, nor how this would combat the problem that I can accidentally spoil my ballot by miscounting the number of choices I've made. An easy mistake to make - even easier if you have a condition like dyslexia.
This seems like the best solution - easy and transparent for voters, and then spoiled numbers can be distinguished from valid ballots.