Let's go through this step by step.
For your plaster outer portion (mold), I recommend at least 8 pieces (2 pieces if it is single-use and you can break it up, in which case make sure to use a soft plaster). And you are going to put a styrofoam of balloon center in it when you go to cast.
- Make a 2 piece blank (master) of your shape around a solid center sphere
- Pour your plaster in two sections
- Open it and remove your blank and your solid center
- Put your styrofoam ball (or balloon) and plaster mold back together
- Pour your cast.
This does depend on how hot your clay is. If it is hot enough to distort the styrofoam sphere, replace styrofoam with a water or sand filled balloon instead. Then you can pop and drain it afterwards. Don't use an air filled balloon - it will compress, distort and shrink with the weight and heat of clay.
Also, the thicker your final shape the more pieces your mold may need, especially if you intend to reuse it. Otherwise it will lock around your shape and you will not get it out without damaging one or the other. 8 pieces is best for a reusable, think of 8 cubes stacked to make one large cube.
Your blank (also called the master) will need to be get out without damaging the the plaster. This is when styrofoam, sculpting foam, unfired sculpting clay, or any other soft media will work best.