This is a difficult question to answer due to the fact that Garth Brooks does not release his concert earnings publicly. However, we can look at some past reports and estimates to get an idea of how much he might make per show.
According to a report from Billboard in 2016, Garth Brooks made $90 million dollars from touring that year alone. This averages out to approximately $1 million dollars per show. However, it should be noted that this figure includes all aspects of his tour revenue, not just ticket sales. It is likely that a portion of this comes from merchandise sales, meet-and-greet packages, and other sources.
So, while we cannot say for sure how much Garth Brooks makes per concert, we can estimate that it is probably somewhere around $1 million dollars.