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The radio button field is actually designed to only allow your user to select one option.
If you need to select more than one options, then I believe what you need to use is the check box field.
Here's a quick tutorial about it:
1. Add the check box field under the "Basic Fields" section. You can also enabled the "Allow Other" option here.
2. If you explore the properties of it, you can see that it allows you to set the minimum and maximum number of selections that your users can choose:
I hope that helps.