This is the description shown on the website:
When we say free, we mean free. No hidden fees, no commitment, no credit card required. The free edition includes everything you need to design, build, and deploy demos, prototypes, or small applications. It includes a deployment environment for each application with unlimited users. You’ll want to upgrade to a paid plan if you will have more than 50 concurrent users, rely on batch imports of data, or need to use of deep links.
It says UNLIMITED Users
You can find more information on the limitations of the Free Editions on this page https://www.mendix.com/pricing/. I think that’s also where Dilan’s answer is coming from