how much balsamic vinegar should I add at the end?

If you have a really good balsamic, & you want to do a dramatic & fun presentation, you could present the balsamic in a fun way at the table & let your guests add their own. And, I guess that is also to taste so have fun with it! That’s so great that it turned out so well!

The recipe says to add the balsamic vinegar "to taste," so there's no specific amount.
One way to work that out is to look at the recipe which serves about 6 to 8 people.
So ask yourself, how much of the vinegar would you use for each person?
Then add that just before serving.