Count blocks instead of one long sequence
In a 25 m pool, 400 m is 16 lengths. Keeping a number that changes at every wall works until your attention goes somewhere else. Splitting the 400 into four 100 m blocks makes the task smaller: you are in the second 100, then the third, instead of wondering whether this is length nine or ten.
The same idea works for sets. Instead of counting 32 lengths for 8 × 100 m, count eight repeats and four lengths inside each. The workout structure carries part of the memory load.
Update the count at the same moment every time
Use the wall as the trigger. When you touch or push off, confirm the block that just ended or the one that is beginning. Avoid recalculating in the middle of the length, where the count is more likely to mix with breathing or a technical cue.
Some swimmers also attach a landmark to each end of the pool: start of the 100 at one end, halfway at the other, for example. The important part is using one stable system rather than changing methods inside the same swim.
For longer swims, add a concrete landmark
Over 800 or 1,500 m, even blocks can blur. You can use the pace clock to note each 100, move a small object at the wall when the facility allows it safely, or use a watch if it tracks your lengths reliably.
Technology helps but is not perfect. Drills, stops, and unusual movements can confuse automatic counting. Keep at least a rough mental landmark so you know whether the displayed number makes sense.
Information
Lost the count? Return to the last certain landmark
Do not spend the next several lengths reconstructing fifteen previous turns from memory. Restart from the last block you know. If this is a training swim rather than an official distance, being off by 25 m is often less important than getting your attention back on the swimming.
The set itself can make counting easier
If long continuous swims regularly become confusing, create landmarks in the structure. A 1,000 m swim can become 5 × 200 m with very little rest, or 4 × 250 m if that better fits the purpose. You keep sustained work while giving the brain clear checkpoints.
Counting is itself a swimming skill. The goal is not to never make a mistake; it is to use a system simple enough that counting stays secondary to the actual swimming.