By default Mastodon polls only allow people to select one option. However, you can also allow multiple selections.
If you're using your server's website:
1. Write your post and poll as normal, but do not post it yet
2. Click on the circles next to the options, they will change into squares, and if you click again they turn back into circles
3. Use circles if you want a poll where people select one option, use squares for selecting multiple options.
More info at https://fedi.tips/customising-mastodon-polls-to-allow-multiple-selections/