... allow me to introduce some changes to your code.
# instead of using separate variables as counter, let's use a list!
counter = [0,0,0,0,0,0]
n = int(input())
for i in range(0,n):
# we find the max counter
mode = max(counter)
# and we find which number the max counter belongs to
max_roll = counter.index(mode)+1
# The problem with this code though, is that it doesn't care if there are two max rolls.