Yes, the Meraki cloud authentication uses a username-password based system, which is typically configured with PEAP-MSCHAPv2.
The Meraki cloud server's certificate should be trusted by the client for the authentication process to work correctly. However, certain client devices might get a prompt to trust or ignore validation for the Cisco Meraki certificate they get when connecting to an SSID.
The Meraki Dashboard does not explicitly state a maximum limit for the number of users that can be created for authentication. It's important to note that for each SSID, there's a design limit of 128 clients per radio built into the APs (But I really don't recommend connecting more than 30 clients per radio).
The Meraki cloud authentication can integrate with external identity providers through RADIUS or SAML. For example, it can be configured with Azure AD using SAML.
Yes, the Meraki cloud supports certificate-based authentication such as EAP-TLS. This can be easily deployed to iOS, Android, OS X, and Windows clients using the Systems Manager.
https://documentation.meraki.com/General_Administration/Managing_Dashboard_Access/Configuring_SAML_SSO_with_Azure_AD
https://documentation.meraki.com/General_Administration/Cross-Platform_Content/Configuring_EAP-TLS_Wireless_Authentication_with_Systems_Manager_Sentry_Wifi
I am not a Cisco employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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