Sec 252-min no. of directors

As far as i know the min number of directors in public company is 3 and in private company is 2.But article can provide a higher than these limits.In such cases,  if minimum number is not satisfied then the act done by the directors would be invalid.But if articles provide that the the directors can act if vacancy is created in the board, then the act would not be invalid.for example to increase the number of directors if it has fallen below the statutory limit.