A cleaner way to have an NSMutableArray of counters?

How do I make this clean. I know how to make this really simple with a C array or a C++ vector, but I'm trying to learn ObjC! Any help greatly appreciated.

NSMutableArray *count_id = [NSMutableArray new];
// Can this be done better for an initialization?
for (int i; i < 256; i++) {
 [count_id addObject:[NSNumber numberWithInt:0]];
}

NSData *data = [NSData dataWithContentsOfFile:fileName];

for (size_t dgStart = 0; dgStart < data.length; ) {
 unsigned char id;
 unsigned int size;
 [data getBytes:&size range:NSMakeRange(dgStart, 4)];
 [data getBytes:&id range:NSMakeRange(dgStart+5, 1)];

 //
 // Yuck!  I just want to increment the number by 1.  How do I improve this?
 //
 [count_id replaceObjectAtIndex:id withObject: [NSNumber numberWithInt:[[count_id objectAtIndex:id] integerValue]+1 ] ];

 // Jump to the next packet
 dgStart += size+4;
}