For such a "lingua franca" the pure number of speakers is not quite as relevant as the distribution of those speakers. According to the numbers (https://en.wikipedia.org/wiki/List_of_languages_by_total_num...), approximately 14% of the world speaks English as a first or second language as well. More importantly, English has 400 million more speakers who speak it as a second language when compared to Mandarin, which suggests that its speakers are more widely distributed on Earth.
While I agree with your viewpoint and it stands up on it's own, I also think it's important to remember the number of people that know at least 20 or 100 words of English. Simple statements can help at a lot of places, whereas I only know a single Chinese word. The number of people that can say basic numbers in English or very simple statements is much higher.
