Ncisfanwiki currently lists 36 rules. If you've seen the show, you know that the rules are numbered. The rule with the highest number on the aforementioned site is rule 70. Some numbers have multiple rules, for example there are two rules 1:
Never let suspects stay together.
Never screw (over) your partner.
To answer your question, 36 unique rules have been mentioned in the series, some with the same number. If we assume the rules form an ordered list without skipping any numbers than there are more than 70 (because of some rules having the same number).