Initial attacking armies: (Count ALL attacker armies on
attacking country)

Initial defending armies: (Count ALL defender armies on ALL
defending countries)

Number of the defender's territories:

Number of trials to run (more gives a more reliable answer
but takes longer:

Count all attacking armies and all defending armies. Thus, if
an attacker is sitting on Venezuela with 10 armies and the
defender has three armies on Brazil, four on Peru, and one on
Argentina, then you can compute the probability that a
campaign starting in Venezuela will successfully take Peru,
Brazil, and Argentina by entering "10" for
attacking armies, "8" for defending armies, and
"3" for defender territories.