It makes sense to use the minimum number of swims and blood samples which will give you the information you require. Providing the test is performed correctly and consistent protocols are used from test to test, one swim at each step is adequate. Proper performance includes even-pace swimming, controlled application of effort and evenly spaced speed increases, across the range, from step one to maximum. It is not a disaster if the swimmer starts too slowly, but it could be a problem if they start too fast, either by uneven splitting, where the lactate will not reflect the average speed, or by step one being too high an intensity, in which case there will be no information about the lower aerobic levels. This can also cause the swimmer to tire, so that they can't complete all the steps correctly, or show a very tight speed range, which makes it difficult to interpret the data.
Coaches have favourite sets which they use regularly to assess the current status of their swimmers.
These will range from 6 x 1000m, through 10 x 200m, to 8 x 50m. Whatever the distance and repetitions, if the same set is used regularly, then valuable information is gained. The qualities indicated from the results of the chosen set should reflect the targeted qualities of the race model. The T30 has become a very popular swim for training and testing purposes. If swum at an honest, maximum effort, even-paced, it gives results which indicate the anaerobic threshold speed and lactate level for most swimmers.