Nothing your visitors notice. Your quizzes keep running, results keep showing, and every email is still saved.
Extra leads are held from view. Once you pass the leads included in your plan for this period, new leads are stored but hidden in your Leads table, and a banner tells you how many are waiting.
To see them: upgrade your plan and held leads are released immediately. They are also released at your next renewal, when your allowance resets. You can also add a pack of 100 leads for $5, which releases them straight away — see Buying extra leads, generations and smart screens.
Held leads are never deleted, and they're released automatically at your next renewal if you'd rather wait.
See also: Plans and what's included · Leads, export and deletion.