how many simultaneous connections can I make to an 800M controller in 800xA?
For full OPC connection (AE + DA) you are limited by the AC 800M CPU setting "AE Max number of event subscriptions" which is 7.
AFAIK the recommended maximum number of MMS connections per controller is 25 (including controller-2-controller MMS/IAC, process panels, OPC server connections, etc.).
By not connecting the OPC server via the AE protocol, the number of (pure OPC DA) connections could be a bit higher than 7 (but is still limited by the recommended maximum number of MMS connections of 25).