Later experiments are often designed to test a hypothesis that a treatment effect has an important magnitude; in this case, the number of experimental units is chosen so that the experiment is within budget and has adequate power, among other goals.
lˈeɪɾɚɹ ɛkspˈɛɹɪmənts ɑːɹ ˈɔfən dɪzˈaɪnd tə tˈɛst ɐ haɪpˈɑːθəsˌɪs ðˌæɾə tɹˈiːtmənt ɪfˈɛkt hɐz ɐn ɪmpˈoːɹtənt mˈæɡnɪtˌuːd; ɪn ðɪs kˈeɪs, ðə nˈʌmbɚɹ ʌv ɛkspˌɛɹɪmˈɛntəl jˈuːnɪts ɪz tʃˈoʊzən sˌoʊ ðætðɪ ɛkspˈɛɹɪmənt ɪz wɪðˌɪn bˈʌdʒɪt ænd hɐz ˈædɪkwət pˈaʊɚ, ɐmˌʌŋ ˈʌðɚ ɡˈoʊlz.