You can design different mold types, depending upon whether you want to produce a single part or multiple parts from a single mold.
Before designing the mold, including the runner system and cooling system, you should analyze the part to ensure that it's design is suitable. You will then determine whether you are designing a mold for a single part, which is a single cavity mold, or for multiple parts, which is a multi-cavity mold. Multi-cavity molds are used for multiple iterations of the same part, or for multiple parts of the same design, for example, the top and bottom of a remote control in a family mold.
Different tools are available for the type of mold that you select, which can be part only, single cavity, or multi-cavity mold. Careful consideration of the requirements of your part and its mold will result in a higher quality part being produced.
A single-cavity mold has only one cavity, so it can only produce one finished part per injection cycle. A single cavity mold is suitable for producing large parts, or for short part runs.
A multi-cavity mold has two or more cavities. The cavities may be identical, so that they produce multiple copies of a part in a single injection cycle, or they may be different, so that they produce the related parts of a family mold in a single injection cycle.