How do you balance the charge of a half-reaction

I am learning about redox reactions and I am having some trouble with balancing charge of half-reactions. Here is a half-reaction:

MnO4- + 8H+ -> Mn^2+ +4H20

I know that the correct answer is to add 5e- to the MnO4- side, I just don't understand why it is 5. Can someone explain why it's that's amount of electrons? Thank you!

The permanganate ion has an overall charge of 1- Each oxygen is 2-. (except in peroxides when it is 1-). Therefore the total for oxygen is 8- For the overall charge of the radical to be 1-, the Mn
must be 7+.
The overall change for manganese is 5, due to the reduction by 5 electrons.
Therefore the magic number for half of the equation 5.
Use the same system for the other half.