Hey IDV,
You can find the maximum concurrent connections and connections per second on our sizing guide for the MX:
https://meraki.cisco.com/lib/pdf/meraki_whitepaper_mx_sizing_guide.pdf
In an ideal world where all you are doing is splash page logins, as Niksutar mentioned, we'd probably be able to achieve a much higher amount, but in truth the MX is doing way more (NAT, Firewall, Content filter, Authentication, etc.) and therefore I'd rely on the info on the sizing guide and attempt to not exceed them as you might face issues.
You can customise the Splash page independently of which method of authentication you are using by going to Security Appliance > Splash Page . If you create a Custom Theme, you'll be able to modify the HTML as well.
We support all devices, though you might encounter issues with older versions (e.g.: Windows XP, Android 4.X, ...)
Hope this helps!
Giacomo
Please keep in mind that what I post here is my personal knowledge and opinion. Don't take anything I say for the Holy Grail, but try and see!
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