This data is as of day before yesterday from an IBM Vice President who should know what he is talking about. I suspect they found that 24 was too many. Another article I found from about a month ago said seven IT guys supporting IBM's Mac base. Now it's five. . . perhaps they are busier than a one legged man in a rear end kicking contest, but that seems to be the current number.
As for longevity of hardware, my office has found that Macs are like Mack trucks. They just don't break. In almost twenty years of running Macs, we've gone through more than 100 through normal upgrade cycle (we usually replaced them on a four year cycle, although this last one was six) and we've had only two that gave us problems. One was a brand new one that was a lemon just this past quarter, and one that had a hard drive give up the ghost and require replacement. Every single one of the Macs we had was sold for approximately 40% of the original purchase price except for a MacPro which we kept for longer than the normal cycle. It was 30%.
Found a recent article that says 50 Mac support staff for 210K devices. http://www.cultofmac.com/449972/ibm-now-uses-macs-company/