LetterCounts

LetterCounts["string"] gives an association whose keys are the distinct letters in string, and whose values give the number of times those letters appear in string.

LetterCounts["string",n] gives counts of the distinct n-grams consisting of runs of n letters in string.

LetterCounts["string",n,{c1,c2,…}] allows the characters ci to appear in n-grams, in addition to ordinary letters.

LetterCounts[{string1,string2,…},…] gives the counts for each of the stringi.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.