To be brutal about it the question isn't a how many more millionaires. Say Massachusetts has 10 new millionaires. That's great. What if Florida has 1000 new millionaires? Suddenly 10 doesn't sound great.
To my mind it's a revenue maximizing question. Is Massachusetts hitting that metric right? I have no clue. I do suspect that the voters and the people advocating taxes like this in Massachusetts and California are not thinking in those terms. I think they are doing this out of some sense that anyone making that much has to be doing something unfair and that an inefficiency in revenue is just money spent towards fixing that unfairness.
It has dramatically increased revenue, more than expected. Also, if you're the type of person who thinks you shouldn't pay your fair share, feel free to move somewhere else. We have the best schools in the country. We have a strong social safety net. We have a high quality of life. Shit, we live longer than almost any other state. So please, if that's not your thing, take your money and leave immediately.