I count 123 with the stroke on the first length, 2 2 3 on the second, etc. obviously breathing both sides on a 3 count, which you want to do if you don’t already.
Don’t just do massive long swims with no pauses either – do intervals or other things that will make you faster. Just plodding up the pool for ages doesn’t make you get any faster and is a really bad way to train.
Oh and unless you’re super good already (you might well be if you bother doing tumble turns), work on technique with something like the drills described on the swimsmooth website, or the total immersion books, technique makes way more difference to swimming speed than fitness and has massive advantages in triathlon as it potentially leaves you less knackered after the swim.