If you're posting a poll through a Mastodon server's website interface, you can switch between "one answer allowed" and "multiple answers allowed" by clicking the circles next to the answers. When clicked they turn into squares, when clicked again they turn back into circles.
⚫ Circles mean only one answer is allowed, and is the default.
◼️ Squares mean multiple answers are allowed.
(Apps will have their own interface for switching between these options.)
