My understanding is you must add one community lot to your neighborhood each generation.
However, you can add up to however many lots as you have eligible Sims since each Sim can only earn 1 community lot.
JMJ, was giving AbstractSilver the option to let the heir have double since she expressed concerns over finishing the challenge given the high tendency of her game to corrupt saves when they get to around generation 5 or so.
But that's just my interpretation of the rules and the situation that AbstractSilver is in.