Using Mapping Sheets add-on you can plot as many locations as you can fit in a 10 MB (megabytes) data file.
We have seen up to 30,000 locations, however you can hit the 10 MB file size limit with a smaller number, depending on the amount of information you are including on the map (number of columns and amount of data in your sheet)
This add-on create a data file (those *.json files within your Google Drive xsMapping folder) holding all the details that are later shown in the mapping web page. That's the file that cannot go over 10 MB in size.
So far, this limitation has only affected 9 users.
Currently, this 10 MB file size limit is a limitation within Google Apps Script when calling the Google Drive API, either using a Script function or Advanced Drive Services. We opened a support ticket issue with Google around this limitation, that resulted similar with an older one addressing the same problem, even though still open, it looks like that 10 MB limit is not going to be increased.
Other notes around plotting many locations
> You might have to wait a few days to complete your map, if hitting the Geocoding daily quota limit
> As long as the mapping page is using client side browser technology, the performance might be affected with many thousands markers in the map, particularly on the initial loading.
> If you hit and cannot avoid the 10 MB file size limit, you might consider a solution around Google Fusion Tables alternative. Please, let us know if that the case, to discuss how we can help you on that.