Sentence:  Although the United States and the U. S. S. R. have been arguing whether there shall be four , five or six top assistants , the most important element in the situation <WSD>is</WSD> not the number of deputies but the manner in which these deputies are to do their work . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)