I'm terrible at keeping count of how many lengths I've done. Especially when I'm trying to do 16 or 32 without stopping. My mind wonders - more often than not I'm writing blog post like this one in my head - and I just completely forget where I'm up to.
This has always been the case - I have a great memory for poems but for nothing else - so I have a system to deal with it. When I forget how many lengths I've done, I take 4 from the number that I think I've done and carry on from there. Eg. if I think I've done 24 but I'm really not sure, I'll go back down to 20 and carry on from there. That way I might end up doing too many but I'll hopefully never end up doing too few. It also gives me a good reason not to forget.
ps. this was not helped at all by the fact that I recently swam in a 33m pool (I don't think I'll be making it my new base).