Polls use credits based on their length, just like regular messages. A message under 160 characters will generally cost 1 credit per recipient.
When you're creating your poll, check the character counter below the poll question to see how many credits it will use. This includes the question and answer options.
If you're including your poll in a message (as opposed to triggering it with a keyword), the counter in the Create Message window will show the total character and credit count to send your text, poll, and poll's answer options.
Note: When someone responds to your poll, you're not charged any credits for their answer or the "Thanks for voting" auto-response.