Here's a post that might help - https://documents.trex.com/is/content/Trex/full-trex-installation-guide-2019pdf.pdf
what climate do you live in?
Check with your local supplier. I was told that joists need to be 16" on center while solid wood like cedar or redwood can get by with 24" on center. The joist material depends on where your deck is. Pressure treated will stand up better than fir and is essential when there is any ground contact.
Trex has to be 12 inches apart on center. It will sag at 16.