The change in colour of the indicator occurs after exactly 20 ml of the Sodium Hydroxide alkali have been added.
The reaction of the alkali with the acid is a neutralisation. Generally, it can be said that acids and alkalis react to form salt and water:
NaOH + HCl -> Na+ OH- + H+ +Cl- (aq) -> NaCl + H2O(aq)
From this formula, you can see that Hydrochloric acid and NaOH alkali react in a 1:1 ratio. An NaOH molecule reacts with an HCl molecule. As a mol (substance amount) always contains the same amount of particles, one mol of Hydrochloric acid is the exact amount required to neutralize one mol of Sodium Hydroxide alkali. 20ml of 0,1 mol Hydrochloric acid is neutralized by the same amount of Sodium Hydroxide. Alternatively, you could neutralize 20ml of 0,1 mol Hydrochloric acid with half the amount of doubly concentrated Sodium Hydroxide.
20ml 0,1m HCl + 20ml 0,1m NaOH
20ml 0,1m HCl + 10ml 0,2m NaOH
Learn more about this experiment: http://www.science-tube.com/index.php?c=chemie§ion=060