That message is not standard in Magento, so you probably have a custom theme. Because of that, I cannot help you by saying file names where you can change this behavior, but I can give you advice just to search all files in app/design/frontend for a specific word, for example, "Only". That's important because on that way you'll be close to the code which deals with that message. There you'll probably have if statement which inspects wheater qty is above zero or not. There you can change your messages.
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