Is it possible to reference a variable with a string and an int?

This is an anti-pattern I call the Poor Man's Array. The better way to do this is to use a proper collection like an array instead of a bunch of variables that are secretly related. Done right, the code with an array will usually be a lot shorter and cleaner too.

From what I can infer you are trying to set/retrieve different variables based on the value of j.

You could use a dictionary for this purpose:

NSMutableDictionary *numbers = [NSMutableDictionary dictionary];
int j = 1;
[numbers setObject:[NSNumber numberWithInt:3] forKey:[NSNumber numberWithInt:j]];

And then to retrieve:

[[numbers objectForKey:[NSNumber numberWithInt:j]] intValue];

It's a bit verbose, but you could simplify it by creating a small class.