replaceOccurrencesOfString:withString:options:range: will return the number of characters replaced in a NSMutableString.

[string replaceOccurrencesOfString:@"A" 
                        withString:@"B" 
                           options:NSLiteralSearch 
                             range:NSMakeRange(0, [receiver length])];

Whenever you are looking for things in a NSString, try using NSScanner first.

NSString *yourString = @"ABCCDEDRFFED"; // For example
NSScanner *scanner = [NSScanner scannerWithString:yourString];

NSCharacterSet *charactersToCount = [NSCharacterSet characterSetWithCharactersInString:@"C"]; // For example
NSString *charactersFromString;

if (!([scanner scanCharactersFromSet:charactersToCount 
                          intoString:&charactersFromString])) {
    // No characters found
    NSLog(@"No characters found");
}

// should return 2 for this
NSInteger characterCount = [charactersFromString length];

Performance comparison for the different Objective-C solutions.

Assume that all the methods below are NSString extensions (inside @implementation NSString (CountOfOccurrences)).

As a sample, I've used a random generated string of length 100000000 using all Latin characters (CharacterSet(charactersIn: "\u{0020}"..."\u{036F}") in Swift). And the character to count was @"a".

Tests performed on Xcode 10.3 on Simulator in release configuration.

Fast solutions (exact character-by-character equivalence)

There are two ways to count for a character: using NSLiteralSearch or not. The count will be different and the performance will be fundamentally affected. For fastest results, we will perform exact character-by-character equivalence. Below four solutions give very close performance results.

1. Fastest solution: an adaptation of CynicismRising answer.

Using replaceOccurrencesOfString:withString:options:range:. This is the fastest solution in all scenarios: even if you replace NSLiteralSearch with kNilOptions, you're still faster than pierrot3887 scanner solution.

- (NSUInteger)countOccurrencesOfString:(NSString *)stringToFind
{
    return [[NSMutableString stringWithString:self] replaceOccurrencesOfString:stringToFind
                                                                    withString:stringToFind
                                                                       options:NSLiteralSearch
                                                                         range:NSMakeRange(0, self.length)];
}

2. Second fastest, another adaptation of CynicismRising answer.

Using stringByReplacingOccurrencesOfString:withString:options:range:.

- (NSUInteger)countOccurrencesOfString:(NSString *)stringToFind
{
    NSString *strippedString = [self stringByReplacingOccurrencesOfString:stringToFind
                                                               withString:@""
                                                                  options:NSLiteralSearch
                                                                    range:NSMakeRange(0, self.length)];
    return (self.length - strippedString.length) / stringToFind.length;
}

3. Third fastest, Jacque solution.

Using CFStringGetCharacterFromInlineBuffer. See https://stackoverflow.com/a/15947190/1033581.

4. Fourth fastest, a conversion of my Swift answer to Objective-C.

Using rangeOfString:options:range:.

- (NSUInteger)countOccurrencesOfString:(NSString *)stringToFind
{
    //assert(stringToFind.length);
    NSUInteger count = 0;
    NSRange searchRange = NSMakeRange(0, self.length);
    NSRange foundRange;
    while ((void)(foundRange = [self rangeOfString:stringToFind options:NSLiteralSearch range:searchRange]), foundRange.length) {
        count += 1;
        NSUInteger loc = NSMaxRange(foundRange);
        searchRange = NSMakeRange(loc, self.length - loc);
    }
    return count;
}

Slow solutions

The below solutions do not use NSLiteralSearch and do not perform exact character-by-character equivalence. The first two are maybe 10 times slower than the fast solutions, and the last one is maybe 100 times slower.

5. Slow solution: adaptation of pierrot3887 answer

Using scanUpToString:intoString:. Too bad that NSScanner doesn't offer an option for exact character-by-character equivalence.

- (NSUInteger)countOccurrencesOfString:(NSString *)stringToFind
{
    NSScanner *scanner = [NSScanner scannerWithString:self];
    scanner.charactersToBeSkipped = nil;
    scanner.caseSensitive = YES;
    NSUInteger numberOfOccurrences = 0;
    while (!scanner.isAtEnd) {
        [scanner scanUpToString:stringToFind intoString:nil];
        if (!scanner.isAtEnd) {
            numberOfOccurrences++;
            [scanner scanString:stringToFind intoString:nil];
        }
    }
    return numberOfOccurrences;
}

6. Slower solution: gbaor solution

Using componentsSeparatedByString:. Regarding the argument of doable in one line, note that the fastest solution given above is also a one liner.

- (NSUInteger)countOccurrencesOfString:(NSString *)stringToFind
{
    return [self componentsSeparatedByString:stringToFind].count - 1;
}

7. Slowest solution: adaptation of vikingosegundo answer

Using enumerateSubstringsInRange:options:usingBlock:.

- (NSUInteger)countOccurrencesOfCharacter:(NSString *)characterToFind
{
    __block NSUInteger counter = 0;
    [self enumerateSubstringsInRange:NSMakeRange(0, self.length) options:NSStringEnumerationByComposedCharacterSequences usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
        if ([characterToFind isEqualToString:substring]) counter += 1;
    }];
    return counter;
}

I would probably use

NSString rangeOfCharacterFromSet:

or

rangeOfCharacterFromSet:options:range::

where the set is the set of characters you're searching for. It returns with the location of first character matching the set. Keep array or dictionary and increment the count for character, then repeat.

The example with the Scanner was crashing on iPhone. I found this solution :

NSString *yourString = @"ABCCDEDRFFED"; // For example
NSScanner *mainScanner = [NSScanner scannerWithString:yourString];
NSString *temp;
NSInteger numberOfChar=0;
while(![mainScanner isAtEnd])
{
   [mainScanner scanUpToString:@"C" intoString:&temp];
   numberOfChar++;
   [mainScanner scanString:@"C" intoString:nil];
}

It worked for me without crash. Hope it can help !