The number of cumulus—oocyte complexes (COCs) in a droplet is known to affect oocyte maturation rates. In this study, we investigated whether droplet size or the number of COCs examined in in vitro maturation (IVM) affect mouse oocyte maturation, fertilization, or early embryonic development. Moreover, we aimed to determine the optimal, practical culture condition of IVM that could be achieved without changing the culture medium composition. The droplet sizes used were 20, 50, 100, and 200 µl, and the numbers of COCs examined were 1, 5, 10, 20, and 50. The groups with oocyte maturation rates exceeding 75% were treated with in vitro fertilization and in vitro culture. Compared with in vivo-matured oocytes, the 20 COCs/100-µl group did not exhibit significant differences in developmental competence and quality. We also observed progression to blastocysts in the 5-COC group, but the zygotes were likely to form multiple pronuclei. The results indicate: (1) droplet size and numbers of COCs examined in IVM affect oocyte quality; (2) the 20 COCs/100-100-µl condition effectively stimulated maturation in all experiments in this study; and (3) culture of 5 COCs in IVM can also produce blastocysts.