I don't really know why you would use an array to accomplish such thing, i would just save an integer in NSUserDefualts like this.
-(void)increaseScreenshotCount{
//this will work even the first time when your key does not exist
//get the value from UserDefaults and increase it ++
NSInteger CurCount = 0 ;
CurCount += [[NSUserDefaults standardUserDefaults] integerForKey:@"ScreenShotCount"];
CurCount++;
[[NSUserDefaults standardUserDefaults] setInteger:CurCount forKey:@"ScreenShotCount"];
[[NSUserDefaults standardUserDefaults] synchronize];
NSLog(@"Current Screenshot Count %d", [[NSUserDefaults standardUserDefaults] integerForKey:@"ScreenShotCount"]);
}