How many board members should we have?

Who decides how many board members? Our CC&Rs say the board 'consists of 3-5 members.  There is no specific guidance on who gets to decide whether it is 3, 4, or 5.

We have had 4 members for a few years now. Last year, one person volunteered to fill the 5th spot (through the election), but the spot was not filled. Again this year, they are just saying there are 2 positions up for vote, but there are 2 members up for re election and the seat that has been empty for years.