I'm not sure what you mean by each modem has a different uerID; you mean that if you dial 8 times to the ISP, you have to use 8 different names to authenticate to the ISP?
In simplist terms, you would use a dialer-rotary group to configure everything, to include 1 dialer map. Then assign each async-interface to that rotary-group. If something more complex is required by the ISP, you can configure each async-interface independently (use the same unnumbered interface for an IP address). As long as the ISP presents the same name, we should bundle all the interfaces together directly.
If you have any issues (either modem, PPP, dialer, etc), then you might have a look at "debug dialer", "debug ppp neg", "debug modem", and "debug chat".