Digital signatures involve a variable number of parties depending on the signature hierarchy and PKI structure. Basic scenarios involve 2 parties (sender signing, receiver verifying), but with certificate authorities, registration authorities, and timestamp authorities, there can be 3, 4, or more parties. Some implementations use just sender and receiver (2 parties), while enterprise PKI systems involve certificate authorities, registration authorities, policy servers, and timestamp services. Therefore, the number depends on the specific signature hierarchy deployed.