I happened to a strange problem when I conduct muti-threads with jmeter against web service. It's okay with 100 threads while some of request are failed with 150 threads, which no correct response data render and a warn message "jmeter.visualizers.ViewResultsFullVisualizer: Could not decode response java.io.UnsupportedEncodingException: text/xml; charset=utf-8 ". Does anybody meet this problem? How to resolve it?
BTW, How many threads are the best choosing based on your practice?

Many thanks,