"Lap counters" are used in longer distance swim events, thus,
A lap Is one length of the pool no matter the length of the pool.
At USA swim competitions long distance races have "Lap Counters", a flip card of odd numbers. When the swimmer approaches the wall opposite the starting block wall for the first turn you show the number 1, then 3, etc. For a 500 Yard race there are 20 laps. For a 1000 yd race there are 40 laps to be swum/counted.
So 50 yards is 2 laps in a 25 yard pool. 25 Yard or 25 Meter pools are considered Short Course.
The Olympics are held in a Long Course meter pool which is 50 meters long.
One lap is 50 meters. So 400 meter freestyle has 8 laps, the 800 meter is 16 laps etc.