Formstack will work on any edition of Salesforce, from Group to Unlimited Edition. However, it is important to keep in mind the API limits when deciding how to deploy your forms.
What constitutes a licensed user of Formstack?
A user of Formstack is anyone that needs to create or modify forms for your organization. So if you have one person on the team responsible for creating or modifying your forms, you’ll only need one subscription.
Form submitters do not need a license, nor do users in your Salesforce account accessing the records created from submissions.