Is 5 a maximum number of buildings in Stone Age?
No, there is no limit to the number of huts a player can have. The space on the board is simply done that way for design; you do not have to put 1 hut in each space.

From the rulebook, page 7:


  The player places the acquired building on one of the corresponding spaces of their
  board. Once a player has more than 5 buildings, he stacks them on these spaces.