From Useless Sexual Trivia: Tastefully Prurient Facts About Everyone’s Favorite Subject*:
“[T]he number of human ova necessary to repopulate the world could fit into a chicken egg.”
The human ovum appears to be roughly spherical:
The diameter of an ovum is ~120 µm.
120 µm to mm = 0.12.
The radius of an ovum is 0.12/2 = 0.06.
Applying the formula for volume of a sphere yields this.
The volume of the egg of a domestic chicken is 50.95 cm^3.
This value converted from cubic centimeters to cubic millimeters:
50.95 cm^3 to mm^3 = 50,950 mm^3.
The volume of a human ovum divided into the volume of a chicken egg is this.
So, the number of human egg cells that can fit into a chicken egg is ~56,312,000.
WolframAlpha says the human ovum is 500 µm. However, several other sources give a number closer to 120:
pg. 321 of this.