Every survey you create is one of three types, chosen when you create it:
No purchase required. Capped by admin-configured limits (maximum questions and maximum responses per survey) so you can try the product before subscribing.
Governed by your active plan. Your plan sets limits like total surveys, questions per survey, and responses per month - upgrade any time from your Billing page to raise them.
No subscription needed. You prepay for a bucket of response credits, and each completed response consumes one credit. When credits run out, the survey stops accepting new responses until you top up - it never goes over budget on its own.
You can mix survey types freely on the same account - a subscription doesn't require every survey to use it.