How does one determine how many cores and RAM they need on a server?

There's no exact formula - only rough estimates which have to be based on past monitored and understood usage metrics. There are 3 sticky threads and one Centmin Mod Insight subforum thread below that have a lot of tools mentioned in them that bring such metrics to light

• https://community.centminmod.com/threads/how-to-troubleshoot-high-cpu-load-issues.22483/
• https://community.centminmod.com/threads/linux-monitoring-troubleshooting-tools.2794/
• PHP-FPM performance in high concurrent user work loads is also dependent on number of CPU cores/threads servicing PHP-FPM https://community.centminmod.com/threads/how-to-troubleshoot-optimize-php-fpm-server.15317/
• cminfo top, phpmem, phpstats, sar-json and sar-cpu-interval commands from https://community.centminmod.com/threads/update-cminfo-cmd-with-top-related-server-statistics.14597/

Yes CPU load average 1.0 = 100% CPU usage per CPU thread/core. So on a 8 CPU thread/core server, ideal load max is 8x1.0 = 8.0 or 800%.

For Centmin Mod LEMP stack usage, ideally, you want at least a minimum 2 CPU cores/threads to give Nginx, PHP-FPM and MariaDB MySQL an adequate share of CPU time between them. Though for very light usage 1 CPU core/thread is fine. For forums like Xenforo depending on your forum activity and usage at least 2-4 CPU core/threads to start with and move up based on your monitored cpu/mem/disk I/O server resources usage.

If you need more exact recommendations, my day job is doing paid server optimization consulting which involves analyzing a client's server resource usage and optimizing their server and also advising on web host and server hardware specifications to meet the client's needs including future growth capacity planning Though my rates aren't cheap, they start in 4 figures $$$$ per server.