Why does the equation for photosynthesis (6CO2 + 6H2O → C6H12O6 + 6O2) have 6 of each molecule?

Good question! This is the -balanced- equation for photosynthesis, meaning there are the same number of molecules on both sides of the equation.
During photosynthesis, carbon dioxide and water are converted into glucose and oxygen.
Glucose is a large molecule with 6 carbon molecules (C6H12O6), therefore the smallest number of carbon dioxide (CO2) molecules required to make one molecule of glucose is 6!