You should consider reviewing this document to answer your question:
http://www.cisco.com/univercd/cc/td/doc/product/core/7206/port_adp/config/3875in.htm
The type and number of port adapters you may install in your router may be influenced by what NPE / NSE you have, how much RAM you have, and the version of IOS you are using. Since those details were not provided in your original question, I cannot give you a reliable answer.