Step One - Look up your HEPA unit's CADR
https://docs.google.com/spreadsheets/d/17j6FZwvqHRFkGoH5996u5JdR7tk4_7fNuTxAK7kc4Fk/edit?usp=sharing. Be sure to pick your country at the bottom of the sheet. Thanks to CleanAirStars for creating this spreadsheet. The spreadsheet lists CADR in Metric, (LPS, Cubic meters/hour (M3/hr)) and in CFM, cubic feet per minute. It also provide information about energy use and filter replacement costs.
Step Two - Math
If the CADR of your HEPA is greater than the required CADR, you are good. Always run it at as high of level as you can tolerate sound wise since air mixing is important. If the CADR of your HEPA unit is less than the required CADR, you need additional units. If you are planning a gathering in your home, you might want to make a Corsi Rosenthal box. Large, very effective cheap filters can be made with a less than 10 year old box fan and 4 MERV 13 furnace filters and can be disassembled for storage. Smaller units can be made with Black and Decker 9 inch fans and 12 inch Merv 13 filters. Super quiet filters can be made with Arctic P14 computer fans. Putting filters up higher is better since exhaled breath is usually warmer than ambient air and keeps them out of the reach of children. More filters are better than few since they provide better air mixing in the room and when run at a lower speed, are quieter.
If the weather outside is tolerable, opening two windows will decrease CO2. A fan blowing out one window will increase air flow.