Ninja hit the nail on the head!
Before you decide on how many HEPA filters to be used, there are other parameters namely, which cleanroom class are you looking for (ISO Class 5, or 7, 8 etc.), what is the process to be employed in the cleanroom, does that process generate particles and is it heavy or moderate or low; number of air changes needed to keep the room "clean" (meet the particulate and if required, microbiological; type of HEPA filter; type of filtration etc.
Your cleanroom experts will have all this info. And as Ninja explained, it is paramount that the quality of air must meet the requirements.