SGA size vs SWAP space in Oracle RAC
Typically:

Server Configuration Checklist for Oracle Database Installation

Swap space allocation relative to RAM (Oracle Database)

Between 1 GB and 2 GB: 1.5 times the size of the RAM
Between 2 GB and 16 GB: Equal to the size of the RAM
More than 16 GB: 16 GB


Then there is Solaris...

Server Memory Minimum Requirements


  Note: 
  
  On Oracle Solaris, if you use non-swappable memory, like ISM,
  then deduct the memory allocated to this space from the available RAM
  before calculating the swap space. If you plan to install Oracle
  Database on systems using DISM, then available swap space must be at
  least equal to the sum of the SGA sizes of all instances running on
  the servers.


It is about summing the SGAs of multiple database instances on the same server, not about multiple database instances in a cluster on different servers (RAC).