Two people can own NSC certificates jointly, but not more. There are three types of joint certificates, -- Single holder Type, Joint A Type & Joint B Type.
Single holder certificates, as name suggests, can be issued only to one individual, but placing a nominee is allowed. However, the primary decision maker regarding the certificates will be the holder only.
Joint A Type certificates are issued to 2 people, and on maturity, both receive the corpus together. For any action related to NSC like cancellation or transfer or certificates, changing of nominee, etc., signatures of both holders are required.
Joint B Type certificates are similar to Joint A Type certificates, except that at the time of maturity, the corpus is withdrawable by any one of the Joint Holders, and not necessarily both.