That depends on a lot of factors. Like how big your database size is in comparison to your RAM, the ratio of logged-in to anonymous traffic, whether you have plugins that keep your Sidekiq queue busier, if you are running YJIT, etc.
What is simple is looking at the MiniProfiler data during your peak hour and browsing the forum to see if the performance is any worse and identifying the bottleneck.
Since this CPU is an older one, I’d start with more than the usual number of Unicorn workers, as every request will take longer than usual. But if you are running PostgreSQL and Redis on the same server, you can’t starve those off by running too many workers.
Try running 16 workers to start with, and evaluate how the site is performing.