It's unlimited by defaut but if you are using splash page then in dashboard select network -> Wireless -> Access control (select SSID) -> Advanced splash settings -> Simultaneous logins -> select limit to one user at a time (or) Allow simultaneous devices per user.
![image.png image.png](https://community.meraki.com/t5/image/serverpage/image-id/36652iF4C3C719BAFD57D8/image-size/medium?v=v2&px=400)