Hello,
MS-MPC or MC-MIC is not required for vanilla BRAS (or LAC or LNS) functionality.
Captive Portal functionality can be done on Routing Engine and also does not require MS-MPC/MS-MIC.
However, if You also plan to enable CGNAT and/or DPI and/or URL filtering then You need MS-MPC or MS-MIC.
How many - it depends on the volume of traffic that needs to be CGNATed/DPIed/URL filtered.
HTHY
Thanks
Alex