The 2013 iMac can support up to 32GB of RAM. A quick search on OWC (Macsales.com) USD $147.99 for four 8GB modules, or $77.99 for 2 8GB modules to add to your existing 8GB which will bump it up to 24GB.
Note... the big impediment to your Mac's performance will be the storage device. If you have a conventional hard drive (not Fusion/SSD), then additional RAM will not really increase the performance of your iMac. The conventional hard drive is just too slow.
If you have an SSD or Fusion Drive, then adding RAM will help.
I have a friend I work with that is still using his 2013 iMac as his primary workstation. The RAM was maxed to 32GB and the internal hard drive was removed and a 1TB SSD was installed in its place. Works great. It's not a 5K monitor like more newer iMacs, but it does the job.