I would like to how to determine the number of sampling locations for airborne viable particles monitoring. Are there any things that need to be considered if there is not any straightforward equation?

I would like to know if it is OK to use use N = sqrt (A) for viable airborne particle monitoring.

Formula N = sqrt (A) is obsolete, it is not in ISO14644 since 2010 year update !

I would rather followed Table A.1 in recent ISO14644-1 for establishing number of sampling locations.
According to Table A.1 you will need more sampling locations than by using previous formula.

For example:
An area of 36 m2 requires minimum 9 sampling locations acc. to Table A.1, and not 6 locations as per old calculation N = sqrt (A).

Determining the number of sampling positions for airborne viable particles does not have to be performed in accordance with ISO-14644. But must always be risk based and commensurate to criticality of operations being performed in the room.

As part of your initial assessment you should consider 1) Areas where product/direct product contact areas are exposed , 2) high traffic areas (or areas where operators stand sit for an extended period of time), 3) Location of return air vents (theoretical worst case position for airborne particulates) 4) Areas shown have turbulent airflow (via airflow visualisation studies), 5) Areas where critical operations occur.

These decisions should be documented with each sampling position fully justified based on the above criteria as an example prior to qualification. If you take the above factors into account you will not have to worry that you have the right number of samples in each room, but rather you know that you have right samples in the right positions relating to risk to product and/or worst case/problematic areas.

A competent regulator/auditor/QA officer will not be given confidence if you say you used ISO-14644 as a guide because a) it isn’t a regulatory requirement for viable particles and b) you haven’t demonstrated at all that you’re aware of the performance of your room, the critical processes and inherent risks associated with airborne particulate contamination.