3DS Max will prompt you with a choice to keep scene materials anytime you delete a material from the editor. The material editor window is for editing materials and isn’t designed to be a material manager.
The number of material editor slots DOES NOT limit the number of materials you can have in your scene. This effectively lets users choose between 6, 15, or 24 material editor slots. To change the number of material editor slots, change the selection in Material Editor > Options > Options > Slots settings area. The Compact Material Editor has 2 x 3 material slots by default but allows 3 x 5 and 4 x 6 displays as well. You can have as many materials in this view as you like. This has become the standard interface for many 3D programs today. The latter uses a drag-and-drop, node-based, visual material creation system.