Limit number of responses in a survey created in FORMS

There isn't a workaround except to clear out previous responses, so you can continue to collect more responses: 

"If you require more, we recommend exporting existing responses to an Excel workbook, and then clearing these from your survey or quiz. This will enable you to collect more responses once cleared."

Is this limit per survey/quizz I wonder? Further clarification would be helpful.

The best way to get Microsoft to increase this limit is to pursue this on the https://microsoftforms.uservoice.com/forums/386451-welcome-to-microsoft-forms-suggestion-box.  Create an idea and get some support behind it, as that limit is bound to be a deal breaker for other customers as well.  

Alternatively, navigate to the upper right corner of the Microsoft Forms window, choose More Options (...), and then select Feedback and do it that way.  Microsoft does listen to this feedback and takes it on board.

You may find these limits may be increased or made clearer when the service goes GA for commercial tenants anyway.