Your question prompts more questions! What is the current legal form of your company? LLC? Sub-Chapter S? Chapter C? How many Board members do you currently have? Are they founders? How much ownership does each have? How long has your company been operating? Are you pre or post revenue?
As you will detect, there is no one size fits all! Generally, each Board member has one vote. Board members are generally elected by the owners of the company (shareholders, unit holders...).
What are the catalysts and purposes to substituting or expanding the Board - investment? business growth? Who are the Board candidates and how are they connected to the you and the company?
Once you answer these questions, you will understand the possible future Board composition options better.
A few general guidelines - keep the Board small -3 or 5 is a good number until you have significant revenue and investment. Board responsibilities are series - are the candidates understanding and up to the challenges ahead? If investors - would observer rights be acceptable?
And, I shouldn't have to say this... many early stage companies run into trouble or fail because their founders do not know when to tsp back from management to become good owners and guardians of their company. Ask yourself if this is you. If yes, take steps accordingly. If no, good. Either way build a strong, strategic board tapping into the resources and capabilities new members willbrign to the success of your company.