I wouldn't say so. I for example tend to just reciprocate the amount of xs the other person likes to put. If they put three, I'll put three at the end, I don't know why it's just a subconscious thing I realised when I thought hard about it. The only exception to this is, occasionally you'll get some people (esp boys) who never put any xs, maybe cos they're not used to doing it to their male friends, and I still always put at least one just for courtesey, even if they don't put any at all.
So to sum up, no, it might be that she always puts that number so he is just being polite and doing it without thinking. If he does like her I doubt that is going to be the only sign.