Limit of 500 responses per free accounts (not clear to me)

Hello everyone,
I have a free Qualtrics account. I have read on the official Qualtrics website that I am subject to a response limit of 500.
The issue I'm having is that I don't understand this limitation. Do I get:
a) 250 per survey;
b) 250 responses across all surveys;
Does anyone know for sure? I have a survey to run very soon so I need to understand as soon as possible if I can use Qualtrics or not, since I have to collect around 250 responses.
Thank you very much.
S.