A rep will be defined as what you performed against the wall, C2W or B2W, against a spotter, or freestanding. It is the drill, performed once.
Sometimes, we will want to execute the given drill several times in a row. In that case, we will perform several reps successively, within one series, or set.
In handstands therefore, some sets are comprised of only one reps: you set yourself up, start the timer and perform the drill once. For instance, if you try to hold yourself for 20 seconds against the wall, that is a set of 1 rep x 20 seconds. Another example: if you try to freeze before you land in your kick-up practice, the set is over by the time you reach the wall. You only have one chance, one rep, to do this right.
On the contrary, you will be asked to do several reps of ping pongs (balance) or good dog bad dog (alignment) in each set.