STOCKHOLM—Spotify is planning to hire hundreds of staff to boost its advertising sales in Europe and elsewhere, as the music streaming service looks to increase revenue from customers who don’t pay a monthly fee but make up the bulk of its user base. “We are increasing our ads business marketing workforce by over 70 percent in Europe, Australia, and Canada … and that’s off a pretty sizable base,” Lee Brown, Spotify’s head of advertising business, said in an interview. “We’re investing in our advertising business. As far as a long-term strategy, I think gone are the days of advertising being less than 10 percent of our overall revenue.” Spotify has also hired an ad industry executive with 25 years of international experience to lead international sales, Brown said, though he did not give a name. The company, which earns income from paid subscriptions and by disseminating ads to non-paying users, …