What is your definition of sentience? I've seen it defined as "having feelings" or as "having feelings and self-awareness". I don't see why the ability to add numbers should have anything to do with that. Some animals that are widely considered sentient have some sense of numbers and counting, but many cannot do addition. And many young children can add small numbers but not large numbers. I don't think this is a good measure or indication of sentience at all. Determining whether something actually has feelings or is just giving the outward appearance of having feelings is a very deep or possibly even an intractable problem.