Research on polyembryony suggests that the presence of multiple embryos in a seed confers an advantage for seedling survival. Because observations from embryo to seedling stages are lacking, however, the effect of the exact number of embryos on seedling survival is unclear. In this study, we evaluated the effect of seed embryo number on seedling survival and growth to determine the number of embryos in a seed that are advantageous for seedling survival in Ophiopogon japonicus (Thunb.) Ker Gawl. var. umbrosus Maxim., which is a taxon exhibiting cleavage polyembryony. We also investigated whether seed mass affects seed embryo number and seedling survival and growth. We found that the number of embryos in seeds of O. japonicus var. umbrosus was weakly dependent on seed mass. As the number of embryos increased, the number of seedlings surviving from seeds initially increased and then decreased; the greatest number of seedlings was produced from an intermediate number of embryos, with the number of embryos producing the greatest number of seedlings increasing with seed mass. The sum of individual seedling lengths increased with the number of seedlings. Our results indicate that an intermediate number of embryos may be advantageous in polyembryonic O. japonicus var. umbrosus.