To unlock the Leisure Suites trophy in Cities: Skylines, you will need to to have a total of 1,000 squares of leisure specialized commercial zone.
Simply setting the squares as leisure specialization isn't enough, the squares need to be built upon, so this is something that will come later in the game, even though the leisure specialization unlocks once you reach Busy Town level at around 1,200-5,000 population.
This will be easiest if you have a large city, make a separate save of it, and then draw one huge district across the whole map and set it as leisure specialization. You should begin to see the commercial zone buildings being demolished and replaced quite quickly as there will be lost of demand from your high resident count.
Fast forward time a while until the trophy unlocks.