Probably the most common question in any swimming workout is this, "How many have we done?" Depending upon the coach, a swimming workout can be a mind-bending math test combining ...
Number of rounds,
Alternating stroke, kicking, drilling,
... and more
If you add onto that complexity the wondrous meditative trance that swimming can induce, it's no wonder that swimmers spend much of their rare time on the walls asking each other about the precise count of where they are.
At least when swimmers get to racing longer pool events, they get lap counters, but even those don't stop the best of swimmers from zoning out and losing track of where they are. One of the most famous recent examples of this came in the 2012 US Olympic Trials when now American-record holder and 2016 Rio silver medalist Connor Jaeger flipped at the finish of his 1500, swimming an extra 100 meters. Fortunately, he was fast enough to make it back to the finals and went on to make the Olympic team.
My only bit of advice for life with swimmers is this: don't ask them to count.