The Poll Settings tab under the green Poll Settings tab contains settings which specifically apply to polls: enabling or disabling Unlimited Voting and enabling or disabling a Poll Vote Limit.
Enabling this setting will allow your voters to cast as many votes as they wish. If you are running a contest or election where each voter should only vote once per question, you should disable this setting.
Poll Vote Limit
This setting is also disabled by default. Enabling it will allow you to set a maximum number of votes on your poll. Once you receive that many total votes across all your questions, your poll will close and will not allow any more votes to be cast.
Once enabled, you can set the vote maximum in the box below the switch. Type it in and press Save. You'll see a green box appear indicating that your limit has been set.
More Information on Polls & Settings
Check out these articles for more info on Polls & Settings: