To determine how many total full Salesforce users you have, go to Setup > Company Information. In the User Licenses table, look for Salesforce and Total Licenses (not Used Licenses).

For 1 - 100 Total Licenses, you need the base package.

For 950 Total Licenses, you need the base package plus 9 add-on packs of 100.

When combined with the appropriate permission sets, there is no limit to how many users can administer Simple Survey or view and report on survey data.

Customer, Partner, and Integration users do not count towards Total Licenses, unless you have unusual requirements to provide access to survey data for these users.