The other responder answered how many votes are considered extras, but I’m assuming you may be wondering about how those extra votes get allocated. If so, they would be divided up proportionally based on the 2nd choices of all the voters for that candidate or party. E.g. if Armchair won with extra votes, and all of Armchair’s voters 2nd choices were: 50% for Couch, 30% for Loveseat, and 20% for Stool, then the extra votes would be divided up to match those percentages.

This is a tangent, but when talking about STV I have seen at least one person comment that it does not solve for strategic voting, noting that if you liked a candidate but also knew that:

1. they were very popular and likely to win,

2. you had a rough idea of how the surplus votes would be split (possibly from prior polling),

you could make your vote count for more by voting for your 2nd choice (e.g. Stool) as your 1st choice, making your vote for Stool 5 times more potent than if you were just another extra voter for Armchair. I.e. it doesn’t eliminate strategic voting and so could cause distorted/degenerate outcomes. However, that relies on a lot of assumptions and is much less predictable than under First-Past-The-Post. There’s also at least one research article saying that strategic voting under STV is computationally difficult https://sci-hub.st/https://www.jstor.org/stable/41105995

The required votes to win are: total number of votes / number of seats

So if there are 4 spots or seats or whatever and 100 total votes then eat spot requires 25 votes. If one candidate gets 28, then they have three extra.