After a record-high number of U.S. law firm mergers in 2015 and the same pace in the first half of 2016, the number of combinations over the past few months has slowed down.A MergerLine report released by the legal consulting company Altman Weil on Thursday shows there were 12 new law firm combinations in the third quarter of this year, down from 20 during the same period last year.Only one of the combinations involved a Chicago-based firm. On Sept. 12, Chicago-based Kirkland & Ellis LLP, which has 1,700 attorneys …