"The company reportedly has about 5 million daily active users, with 1.5 million of those paying for the service." * https://slack.com/pricing

At least 10 million a month, I assume?

Edit: "Slack has 5 million daily active users -- 1.5 million of whom pay to use the service -- and had $150 million in annual recurring revenue as of Jan. 31." from https://www.bloomberg.com/news/articles/2017-06-15/messaging...