Thank you for your message.
I have not found an answer within out feature matrix ( link text´however Users and customers are two separated things. Customer can be bounded to a user but its not necessary so its possible that you might have more customers then 100 even in the free edition but you will be limited by other things (e.g. 10 products as maximum...). Also this goes for anonymious users which as I noticed are not bounded to any users.Free edition is meant mainly for testing purposing and running an E-commerce site on it can be somewhat limiting.
You can also try the limitation yourself with e.g. creating some script with our API. You can find examples of how to create both anonymious customers and bounded customers in Site manager -> Support ->API examples -> E-commerce