The default poll on Mastodon allows people to select just one option. However, you can customise the poll so that it allows multiple options to be selected.
Log in through your server’s website, and create the poll as normal, but don’t post it yet. Click on one of the circles next to the options. The circle will change into a square, and the poll will now allow multiple option selection.
If you change your mind, click on a square to change it back to a circle, which means single option selection.