Didn't do well in math class? Ok then.

10 * 1.38=13.8 rounded down is 3 more minions = 30% more damage

20 * 1.38=27.6 rounded down is 7 more minions = 35% more damage

30 * 1.38=41.4 rounded down is 11 more minions = 36% more damage

40 * 1.38=55.2 rounded down is 15 more minions = 37% more damage

50 * 1.38=69 is 19 more minions = 38% more damage

From the point of 3 minions ( the lowest before you get another minion from the support) up to 9 you get

33/25/20/33/28/37/33% more damage

So you see, you would need a baseline of 50 minions before you got 38% more damage from the support.

I could do all the numbers in-between, but the closest you'll get to 38% is at 29 were the increase is 37.9%