I had also heard this from Microsoft tech support, when I was at around 30-40 installs... it started failing around that number, but they couldn't give me a valid reason why it's limited to 25 and also if it would error every time.
Now that I'm around 50-60 installs I have a very generic error every time. I also like how the physical 'Product Key' supposedly isn't the Product Key, it's a Product Pin. Supposedly there's no way to match up all the installs to the individual Product
Keys (which can only be seen when you click on "Install from a disc" --> "I have a disc" --> "View your product key").
I thought the rollout of Office 2013 H&B was annoying at the beginning of this year, enter the Product Key, log into their office.com website, then enter the key again, then download the full install.... but at least this worked. (maybe because I was
under the 25 limit?). Now you only have to enter the key once, but anyone who supports SMBs has been given a huge headache.