We do have a setting to “Show total results” which will show the total number of responses you have received for an individual poll question. It will be displayed on the bottom right of the chart. You can use this to get a count of responses and then chose to move on when it hits an amount. To show the total results on the chart:
Click the Paintbrush icon to open visual settings menu > select Show total results at the bottom of that menu.

Would it be enough for us to allow you to preset an amount and when the “Total Results” nears or hits this number we provide some alert on the chart to notify the presenter?

I’m hesitant to have us try and guess when a poll question has received all of the responses that it will receive. We do not want to try and control the flow of the presentation by pushing you (the presenter) into moving on or remaining on a poll question before you want to. There are also a number of situations where any algorithm we make could have problems. Some poll questions just get higher participation rates than others. Some controversial questions or perhaps funny/icebreaker questions could get widely different participation rates as well as time it may take for students to respond.