Let's say that N alphabetic letters are used. For the first letter of a name we have a choice of all N letters. But, because we can have repeated letters, we also have a choice of N letters for the other letters of the name. From N letters then there are N^3 possibilities for names and N^3≥180. 5^3=125 and 6^3=216. 5^3 doesn't allow for 180 names but 6^3 does, so the minimum number of letters is 6.