It depends on whether you use the static map API or if you use direct access to tiles.
Direct access to tiles is prohibited by Google, so they block you if the server suspects that you are pulling tiles for caching. They do not indicate limitations on this for safety reasons.
Static cards have a limit of 2500 per day per IP address, but it is throttled as to how much you can output per second.
In any case, the use of Static maps or direct access to tiles to save images for later use relates to Google ToS.
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