Single embryo sac is found in an ovule developing from single megaspore, except the case of polyembryony.
Number of ovules present in an ovary depends on the plant species. Some plants have single ovule in an ovary such as mango, wheat, rice , etc. While some plants have many ovules in an ovary such as Hibisus, pea ,etc.
In a flower, single ovary ( Ex:Apple, mango, etc )or many ovaries (Ex: Raspberry , strawberry) can be present. Ovary can be simple ovary (consisting of single carpel) or compound ovary (consisting of many carpels fused together).