https://help.ubuntu.com/community/SwapFaq#How_much_swap_do_I_need.3F

      RAM   No hibernation    With Hibernation  Maximum
      4GB              2GB                 6GB      8GB
      8GB              3GB                11GB     16GB
     16GB              4GB                20GB     32GB
     32GB              6GB                38GB     64GB
     64GB              8GB                72GB    128GB