Hi @vfg_uy ,
I have answered you in the Bing Maps forum as well. The static map API calls other APIs behind the scene so the basic/trial key limitation may be in one of those subsequent calls or it could be just a size limit on the call itself. The doc calls out a pushpin limit of 18 Get a Static Map - Bing Maps | Microsoft Learn so it seems reasonable to assume that a multisided polygon could easily exceed the 18 point limit. I am not saying that is what is happening but I am saying that the API does have limits and likely you will not be able to display 20 or 25 polygons using this API.
UPDATE: I have confirmed with the team that this API does have a 5 polygon limit. To go higher you will need to use a different API / dynamic rather than static map.