One instance of Rapid SCADA supports max. 65535 input channels. If you need more channels, deploy more instances of Rapid SCADA.
The devices can be polled in parallel. You should play with sequential and parallel polling to achieve required performance.
Tested interval of storing data into the archive is 30 seconds. You can set saving interval to 1 second, but it requires additional testing.
Are you sure that your network equipment can provide the required data flow.