5 units from 14 can mean 14-5=9 or 14+5=19.
“At most” means “less than or equal to”, symbolised by ≤.
So n/2≤9 or n/2≤19. That means n≤18 or n≤38.
Since 18 is less than 38 we can say n≤18 because this also satisfies n≤38. In words, the number n is at most 18 in order to satisfy both inequalities.