Pay-per-response surveys don't use your subscription's monthly response limit - instead, each completed response consumes one prepaid credit from your balance.
Buy credits from your Billing page, choosing how many you need at the current per-response price. Credits apply account-wide - any pay-per-response survey you run draws from the same balance.
When a survey's creator runs out of credits, that specific survey stops accepting new responses (other surveys are unaffected) until more credits are purchased. You'll get an email as soon as this happens so you don't lose momentum on active research.