At pretty much any gym you walk into, you’ll be expected to tie a properly retraced figure-8, followed by a stopper knot to prevent it from coming undone. Every once in a while, a climber would go up on a climb, having tied a proper back-up knot and partway up the climb, realize that their backup knot has come undone. If this is a safety thing, then damn if it’s not scary to see one of your two knots totally undone. Beyond that, the question of what’s a proper backup knot comes up time and time again. Is a “Yosemite finish” safe? Does it have to be a double fishermen’s or better? If I just have enough rope for a single fishermen’s, am I screwed to fall to my doom?
The simplest answer is this: there are plenty of knots that do require a backup knot. The bowline is a perfect example of such a knot. Always back up your bowlines. The retraced figure-8, however, is not as long as there is sufficient tail. If you’re looking at a tiny tail, there’s a chance that it’ll slip once the knot starts to tighten on itself. As long as you have at least 6 inches of tail on a standard 9.8mm climbing rope, the knot, if properly tied will not slip.
To answer the other questions, a single, double, triple, quadruple, or any number of fishermen’s will do. A Yosemite finish is also totally safe. Though be exceptionally careful with the Yosemite finish; if you put the rope back through the wrong part of the knot, you can accidentally undo your figure-8.
If you go to a gym and they have a policy that requires you to do a proper double fishermen’s at the end of your figure-8, please don’t argue with them. Although you technically don’t need one, some gyms have policies and things written in their insurance documents that require climbers to use one.