I want to make a isometric tile game in java, like a lot of strategy games out there. What is the best approach to loading the map for this game? Would it be better practice to load one single image for the whole map, or would it be better to make images for each tile and render each tile using a int array?
Its way easier to use a spritesheet. Then just draw a section of the sprite sheet based on an array. This way you only load one image for the entire tilemap. Look into programes like Tiled if you want to tool to build tilesheets.
If you use multiple textures, you will need to switch them in rendering. This process is expensive. You better use a single texture and use a index to represent each tile, with that index and tile size, you can calculate UV Textures to apply in your tile meshes.