How many state ballots can he miss and still get enough electoral votes to win?
Depends on which states
It’s not how many states, it’s which states. Most are all-or-nothing with Electoral Votes. He lost Alabama & GA & 20 others last time around, so if he’s disqualified from any of those it doesn’t much matter as few if any would turn for him this time. HOWEVER, getting the ineligibility meme going may stifle support in his favorable states, and losing even one of those to this issue might just cost him the election.