Eleven.
20
Five 9s:(99 x 9 + 9) / 9 = 100
11
11 with 1 remaining 100 - 1 = 99 = 9 x 11
99 plus 9/9 = 100
(9*9)+(9+9)+(9/9) = 100
5
Nine of them
There are exactly 20 Nines. 9,19,29,39,49,59,69,79,89,90,91,92,93,94,95,96,97,98 which totals 18 and the last number 99 has 2 nines and therefore 20 nines in total.
79
9^2+9+9 +9/9 81+9+9+1=100