What Forms actually gives you
Open the Responses tab and there's a switch marked Accepting responses. Turn it off and the form stops taking answers, and everyone who opens the link from then on sees that it's closed. That's the whole feature.
Two things follow from it. The first is that somebody has to be watching. Forms won't count to thirty for you and shut the door at thirty, so the switch gets flipped late, usually after the person you have to turn away has already submitted. The second is that it's all or nothing. There's one switch for the entire form, not one per question and certainly not one per answer.
Add-ons can take the watching off your hands. Several will close a form once a total is reached, or at a date and time you set. If your problem really is a ceiling, one of those is a good answer and you should use it.
Where a total stops helping
Most people asking how to limit responses aren't running a survey with a ceiling. They're running parent-teacher evenings, tutoring hours, clinic appointments, market stalls, shift rotas. The question isn't how many altogether. It's how many at four o'clock.
A total cap can't answer that. Set it to thirty and the whole form closes at thirty, even though five of your six times still have room. Set it higher and forty people put themselves down for the four o'clock that seats six. Either way you spend an evening rearranging a spreadsheet and sending apologies.
What's missing isn't a bigger limit. It's a limit that belongs to a time rather than to the form.
