I prepare my sacred space and mentally prepare myself before I preform a spell. How many spells can I preform in one session or consecutively. Do I have to centre and ground after every spell I cast in a session.

The decision whether to center and ground after every spell is a deeply personal one and can only be determined by the way the energy feels in your sacred space after you cast.  As a beginner, I would certainly recommend that you center yourself after every spell. Until you learn to recognize the ebb and flow patterns of mystical energy around you, centering and grounding after every spell helps keep you from making rookie mistakes.

Especially in the beginning, every spell can feel like a lightning rod, supercharging your karmic energy and making you feel like nothing can go wrong. This is a very dangerous state to be in. It is the reason why bipolar persons are more dangerous in their manic stages. When you feel that great about the world around you, you can accidentally get careless and sloppy. Careless and sloppy when casting a spell can lead to truly ugly consequences.

The number of spells that you decide to cast at any one time is not important, so long as you are fully charged and prepared to cast each spell. If you allow yourself to become too exhauseted between casting or if the spells are particularly draing, mentally or magically, wait until another time before casting another spell. In short, the differentiation comes in learning how much energy you have and how much you can let loose into the universe before needing to recharge your batteries.