This refers to the number of portable air purifiers you may have located in a room. Portable air purifiers help by locally cleaning the air and good ones - ones with HEPA filters - can filter out virus laden particles from the volume of air they process It is advisable to position the air purifier towards the middle of the occupied portion of a room. And make sure that they are not directly blowing air across the occupants. Avoid air purifiers with extra add-ons like UV light, ionizers, etc. Just one with a good filter is a very sensible choice.