Some forms only need a few responses. If you need to, you can limit the number of responses for your forms.
Open a form from Forms and go to Settings > Responses > Limit number of responses. Choose the maximum number of responses you want, and write a message to show people who try to complete your form after you’ve gotten all the responses you need. Save your form when you’re done.
At the moment you can't set an expiry date for a form, so a response cap is the best way to set a cut-off point. You can also archive your form whenever you want to close it.
Now you just have to sit back and let the responses roll in. To find out how to process them, check out the Analyze help topics.