Write a question about the following article: A September 2014 Lucid Talk Belfast Telegraph poll showed that 40.1% of the population supported same-sex marriage, while 39.4% opposed and 20.5% either had or stated no opinion. Of those that gave an opinion, 50.5% supported and 49.5% opposed same-sex marriage. A poll in May 2015 found that 68% of the population supported same-sex marriage, with support rising to 75% in Belfast. A "mass rally", organised by the Irish Congress of Trade Unions, Amnesty International, and the Rainbow Project took place in Belfast on 13 June 2015, with a 20,000 person turnout. A June 2016 poll gave support for same-sex marriage at 70%, while those opposing it at 22%.
How many organizations organized a June 2015 rally in Belfast?