When You Have Finished Balancing The Equation, Which Chemical Or Element Will Have The Largest Number Of Molecules In The Balanced Equation?

You can look at either the Carbon or the Hydrogen first, as they end up in different reaction products. The 3 atoms of Carbon will limit the amount of CO₂ produced, and the 8 atoms of Hydrogen will limit the amount of water (H₂O) produced. The CO₂ will require 2 atoms of Oxygen for each Carbon atom, and the H₂O will require one atom of Oxygen for each 2 atoms of Hydrogen. Thus, a total of 3(2)+8(1/2) = 6+4 = 10 atoms of Oxygen are required. 10 atoms of Oxygen will generally be found as 5 molecules of O₂.

The reaction seems to be this one:
C₃H₈ + 5O₂ --> 3CO₂ + 4H₂O

Oxygen has the largest number of molecules in the balanced equation.

(See also this answer.)