The benefit of MRU order is that you can use it blindly to go to the last-nth tab without having to eye-coordinate with the contents of the tab bar. It becomes an automatic muscle-memory thing.
Usually less than a dozen, though I don’t quite see the relevance. I use MRU to switch between two to four (rarely more) related tabs.
In code editors I often have several dozens of tabs open, and MRU is a crucial usability feature when coding, so I’d say that its usefulness is independent of the number of open tabs.
