There are a total of 4 explorers available in Tap Paradise Cove. Your first explorer, Jack, is free, but to get the other explorers you must pay rubies. Explorers are used to create plots of land where you can build merchants, houses and decorations once trees are chopped.
Otherwise, the only reason to have multiple explorers is if you need to expand rapidly for a Quest.
UPDATE: I have all four explorers in an Android game and I can't see any difference in the trees per block whether I use 1,2 3 or all 4 explorers at once. I guess we'll have to consign this to "Myths and Legends" of TPC...