Yes, the greater the effort (and length) of the swim, the more the brain refuses to do computation or other simple tasks. Before I start a set, what I do is this: take the no. of 100s of yards of the set, double that no., and use that as the no. of LAPS (NOT lengths) to swim.
Examples: Swim a 400. So double the 4 of the 400 = 8 "laps". So the set is done when you return to the starting side of the pool 8 times (easier to count for me than 16 lengths--a smaller no., plus you only have to add a lap every two lengths instead of every one).
Or, swim a 300. So double the 3 of the 300 = 6 "laps". So the set is done when you return to the starting side of the pool 6 times.
Or, swim a 750. So double the 7.5 of the 750 = 15 "laps". So the set is done when you return to the starting side of the pool 7 times.
Or, swim a 1000. So double the 10 of the 1000 = 20 "laps". So the set is done when you return to the starting side of the pool 20 times.
Etc. ...
Where would you want to swim ?