Hi, I only have access to the draft version of ISO18562, in a couple of places, it says "To produce a meaningful result, more than one medical device may be required to be placed in series or measured sequentially.". So I would use this to allow testing more than one unit at a time.
It is common to divide risk into high, medium and low, based on patient risk, high being life threatening or death, medium being risk of permanent injury, etc. Then allotting confidence and reliability to each category. Using a binomial test, a high risk would be 95%/99% giving 299 samples, medium 95%/95% requiring 59 samples and low 95%/90% requiring 29 samples. From your risk files, you should be able to estimate the risk associated with particulate inhalation and hence establish a sample size.
I have seen this used by many medical device companies and generally accepted by FDA provided the rationale is clearly documented.