Besides the power analysis, there are less formal methods for selecting the number of experimental units.
bᵻsˌaɪdz ðə pˈaʊɚɹ ɐnˈæləsˌɪs, ðɛɹˌɑːɹ lˈɛs fˈɔːɹməl mˈɛθədz fɔːɹ sᵻlˈɛktɪŋ ðə nˈʌmbɚɹ ʌv ɛkspˌɛɹɪmˈɛntəl jˈuːnɪts.