I said aim for it. Doesn't mean it is going to happen,
Minion waves spawn every 30 seconds (6 minions)
Minion waves with siege cannon minion spawn every 3 waves (7 minions)
Minion waves spawn AFTER 1 minute and 30 seconds into the game (1.5 minutes)
1.5 minutes = 3 waves, thus
10 minutes normally = 20 waves (-3) = 17 total waves
Out of the 20 waves, there are 6 waves that contain 7 minions (waves 3,6,9,12,15,18)
However, wave 3 is gone because it doesn't exist (minions spawn at 1.5 minutes in) -> Waves 1, 2, and 3 are gone
20 waves (total) - 6 waves (7 minions) = 14 waves (6 minions)
14 waves (6 minions) - 2 waves (first minute of game) = 12 waves with 6 minions
6 waves (7 minions) - 1 wave (last half minute of 1.5 minutes of game) = 5 waves with 7 minions
Result - 10 minutes = (12 waves x 6 minions)+(5 waves x 7 minions) = 72+35 = 107 farm (minions)
So in 10 minutes, the max amount of farm you can achieve is 107
20 minutes - 40 waves
However, we know that 3 waves are gone from the 40 waves (2 of them being in the 6 minions wave, and 1 being in the 7 minions wave)
40 waves consist of 13 waves that contain 7 minions.
Thus, 40 - 13 = 27 waves that contain 6 minions
27 waves - 2 "faulty" waves = 25 waves x 6 minions = 150 minions
13 waves - 1 "faulty" wave = 12 waves x 7 minions = 84 minions
Therefore, in 20 minutes, the max amount of farm you can achieve is (150+84) = 234 farm
This is a copy paste.
But it should show you just how much cs there is and how much you can get.
SO to sum up at 20 minutes you could have 234 farm. With a average of 10 per min you will have missed 54 farm or 8 waves of cs if you average 180 farm at 20 mins.