Since the antigen receptors are typically composed of two variable domains (on two different polypeptide chains, heavy and light chain), there are six CDRs for each antigen receptor that can collectively come into contact with the antigen. A single antibody molecule has two antigen receptors and therefore contains twelve CDRs total.
Antigen receptors typically comprise two variable domains on separate polypeptide chains (heavy and light). Consequently, each receptor contains six complementarity-determining regions (CDRs) that can simultaneously interact with the antigen. A single antibody molecule has two such receptors, resulting in a collective twelve CDRs.