My company is looking into using Salesforce Surveys.

Does anyone happen to know if there are any limitations we should be aware of? For instance, is there a limit to how many customers we can send surveys to? I did find out that there is NOT a limit to how many responses we receive.

We would be sending to external customers, so would we need community licenses? I am thinking we would not, as we would make this a public community. Would we need to be worried about mass email sends for when we send out the surveys, since they would not have a community license? 

Any insight would be much appreciated. Thank you!