What's the minimum number of days needed for reporting a "monthly average" of daily satellite retrievals?
Provide a monthly average in every grid cell, and describe how many measurements were used for each cell.

There is no fixed rule for the minimum number of days for reporting a monthly average.  A reasonable threshold will depend on the geophysical quantity of interest y, in particular on how much y varies on short and long timescales.

If y varies from month to month, but does not vary a lot from day to day (example: sea surface temperature), you can probably get away with reporting a monthly average even when only one or two days have cloud-free measurements.  On the other hand, if you're measuring precipitation, your signal will be relatively noisy even if you choose a threshold of 20 days.  And any threshold is going to introduce a bias, because a lack of observation is typically due to clouds.  Although you will get a bias in any case.  In other words, if you only have one day per month you see a gap and in the clouds to observe whatever you want to know, it is a pity to throw it away.  Therefore, personally I choose a minimum of 1 observation.

Critically important though: when releasing the product, add a field that tells the user how many days worth of measurements make up the monthly average in any given cell!  Like that, the user can decide which ones to use and which ones not to use.  More broadly speaking: producers should document everything they do.

For your specific examples: search for the ATBD and/or scientific papers describing the product.  Hopefully, there is a field in the level-3 product that describes the number of measurements per cell.  If the answer is not in there, write to the producers and tell them to redo the product...  You will need this information to accurately calculate a yearly average!