Just like the question "how many programs can you run on a computer at one time", I think it probably depends on the parameters of the system (of course they are somewhat fixed on a HR10). I typically never have more than 30-35 during the heavy TV season, but I kill old ones because too many slow it down, and SPs sometimes don't work for following seasons (info is sometimes configured differently for new seasons). So the best policy, unless you are trying to get into Guiness, is to keep them pared down.
Tivo seems to have a (sensible) policy of telling you what the system will do, and NOT telling you what the theoretical limitations might be. They assume that smart owners are reasonable in their operation of the equipment. If someone is trying to set a record, at some point the system will just not work properly. If your goal is to break the system, you will probably reach it. If your goal is reliability, that's probably not advised.