is there any documentation, guide, or rule of thumb recommending a number of computational nodes based on number of cells and number of DPM particles inside domain?
Check Fluent User's Guide and the Chapter Checking and Improving Parallel Performance.
I would say having 30K cells per node is good trade. Fluent can even scale to much more less number of cells.
For DPM the answer is not so easy as it depends on tha parallel method used for DPM. I would not give any judgment on that here.
Ansys customers with active commercial software licenses can access the
customer portal and submit support questions. You will need your active account number to register.