CPU count minus 2 is the rule of thumb suggested in the help system.
To give you a sense of the problem, we have been running with only 2 CPUs. My VMWare admin had reduced CPUs on all guest systems to 2, as a VMWare tuning move. Arguably, that left us us underpowered, but it has been working well for a long time, even though index threads was left at 3 and queue wait time was only 5 seconds. (Documentation recommends queue wait time of 60 seconds.)
After the update, performance was terrible, so I increased CPUs to 4, reduced index threads to the minimum of 1, and increased queue wait time to 5 minutes. Performance is still bad, but I have decided that it is bearable.
Fortunately, JetBrains helped them find a problem. You can see why I am waiting, with baited breath, for the next release.