There are nowhere near 1945 Java rules in a default installation of SonarQube. You must have installed additional plugins that added extra rules for Java.
If you’re just starting an evaluation or POC, I’d encourage you to begin with only the built-in SonarJava rules in place and avoid adding plugins until you’re sure there’s something extra you need. Many of our customers express frustration with the effort involved in de-conflicting the rules that come from extra plugins from the ones already included. “Less is more” versus “More is better.”
The total number of Java rules is 8.9.1 Community Edition is 639. You need to look in the Rules tab to see all the rules available. What you’re looking at is the Quality Profile which only has a subset of the total rules enabled by default.
The “Sonar way” profile includes the most useful and powerful rules while avoiding activating optional rules that may be considered noisy/strict for some teams. It’s up to your analysis to decide which additional rules, if any, you’d like to activate. Again here, I encourage you to resist the temptation to just activate them all.