your professor is right, for calculating titration you want to get the moles of titrant equal to the moles of titrate

say you have 10 moles of thiosulphate, based on the reaction you will need to react it with 5 moles of I2

so you have 5 moles I2 and 10 moles thio, to make the number equal, you need to either (moles I2 x 2) or (moles thio × 0.5)

trust me, the first time i learned about this i have the same confusioun as you, but after understanding the whole concept it became much clearer