Check your Parallel Preferences
And check the number of workers permitted in your "local" cluster; https://www.mathworks.com/help/parallel-computing/discover-clusters-and-use-cluster-profiles.html
The default permitted is the number of true cores you have -- excluding any effect for hyperthreading. (Hyperthreads that are not sometimes waiting for resources, such as waiting for disk or I/O, can end up making the overall execution slower)
Adding more RAM will not affect the number of workers permitted.
On the other hand, you should expect that workers will take a mininum of 2 Gb each, for the MATLAB image, plus the size of your data.
You should double check: is that 50% that you are seeing 50% of the nominal rate if you were using hyperthreads? Because 50% load measured against hypothetical hyperthreads means that your physical cores are at full load.
