apologies for the long delay in response.
I was hoping for a different answer, but the one offered by our internal teams late last year to a similar question on the Quick Sight Community still applies:
Today, Maps have a limit of rendering 10K locations. The limit is reduced when there is a field in color field well. There is a product feature request to (1) Increase data points support and (2) add clarity in message when limits are applied and the maps show only top X data-points.