There are always four continents on a map as far as the game is concerned. When a map is generated, every non-water tile is assigned to be Africa, America, Asia, or Europe. Every one of them, and all four are always used. If you play Indonesia on a Pangaea map, just go found your spice cities on another part of the supercontinent. It's not a situational trait. There's a gamble in that you might start right next to tiles that are considered another continent and you might not, but it isn't situational.