Maximum number of different substrings in big string

Question

I'm trying to find the maximum number of different substrings of string consisting only of lowercase letters 'a' - 'z', if the length of string can be up to $5000$

Firstly, I was thinking that the number of substrings is $\frac{n\cdot(n-1)}{2}$ but then I noticed that a lot of the substrings will be the same, and also because of the length of the string of most $5000$ we cannot put all the possible strings with length 3 for example.

What is the maximum number of different substrings we can get in one string of length $5000$