Easy answer for a 32GB upgrade is the iX Systems Amazon store. You know it is the correct RAM. See Upgrades section. You can also purchase it elsewhere just match the specs of your current, installed memory.
Others will post about what has worked for them and the amounts of RAM they had success with.
TrueNAS uses memory for ARC and that gets freed easily if the system needs it.
You only really have a problem and need more RAM if the amount available for the ARC would be low. Pretty much ignore the ARC and consider it as Free
Linking Basics info, it may help your understanding.
BASICS
iX Systems pool layout whitepaper
Special VDEV (sVDEV) Planning, Sizing, and Considerations