What MERV rated filter should be used with an Air Scrubber device?

The MERV rating depends on the filter the you are choosing. A MERV rating does not apply to a unit, but rather the type of filter they place in their unit.

The higher the MERV rating, the more filtration that you will get. The lower the MERV rating, the less. We recommend using a HEPA filter on these units. 

For the Pre-filter, we recommend using a MERV 8 rated filter. 

Here is an article from the EPA that might help you to pick the filter you may need.

https://www.epa.gov/indoor-air-quality-iaq/what-merv-rating