This is known as the pigeonhole principle
There are (26)(26) = 676 possible sets of initials
Let's suppose one person has the initials JG
We might have to go through a total of 676 more people to find another person with the same initials
And we might have to go through another set of 676 additional people to find the third person with the same initials
So ....1 + 676 + 676 = 1353 people will guarantee three peope with the same initials