Are There Really 613 Mitzvot? Says Who?

We say that there are 613 mitzvot but this isn't really true.  The truth is that you must engage in numerical acrobatics to arrive at the arbitrary number 613.  If, for example, your calculation includes both temporary and perpetual mitzvot then the number is far greater than 613.

If you're interested in this bit of esoterica (and it's totally okay if you're not) then read on:

"It was axiomatic in Israel that the number of mitzvot is 613, no more and no less.  This number is mentioned in the Tannaitic literature.  Its source is in an aggadic exegesis attributed to Rabbi Simla'i, an aggadist of the second generation of Palestinian Amoraim.  'Rabbi Simla'i expounded:  613 commandments were spoken to Moses.  The negative ones numbered 365, corresponding to the days of the solar year, and the positive ones numbered 248, corresponding to the organs in the human body.  Said Rav Hamnuna:  What is the textual source? 'Moses charged us with the teaching (Torah) as a heritage' (Deuteronomy 33:4)--'Torah' has the numerical value of 611; and 'I the Lord' and 'You shall have no other gods' were heard directly from God.'
Note well, that when Rav Hamnuna felt constrained to find a source in the Torah for the number 613, he had to resort to a numerological hint, a method peculiar to aggadah.  Indeed, Maimonides, in his Book of the Commandments, used this aggadic statement to establish principles concerning the number of the mitzvot, a matter on which depend several prohibitions and permissions, leniencies and stringencies.  Thus did he build the halakhic edifice on an aggadic foundation.
Several Sages expressed misgivings about the number 613.  Judah ibn Bal'am denigrated those who number the mitzvot, and who attempt 'to force their count to equal 613.'  In his opinion, this is impossible, for if we were to count all of the mitzvot, including those that were temporary commandments and those that were intended to endure, the number would be far greater than 613.  'And if we confined ourselves only to those that endure, we would find fewer than this number.'
Rabbi Abraham ibn Ezra wrote in a similar vein:  'Some Sages enumerate 613 mitzvot in many diverse ways...but in truth there is no end to the number of mitzvot...and if we were to count [only] the root principles...the number of mitzvot would not reach 613.'
Note that Nahmanides also expressed doubt as to whether this number was fixed as a matter of law.  'Perhaps we should say that this statement of Rabbi Simla'i is not unanimously accepted but is the subject of dispute.  That is, Rabbi Simla'i enumerated the mitzvot according to his opinion and his reasoning, found them to equal this number, and then founded it on this exegesis.'  Yet, in the end, Nahmanides allowed that since 'this total has proliferated throughout the aggadic literature...we ought to say that it was a tradition from Moses at Sinai.'  pgs 770-771, Heavenly Torah by Abraham Joshua Heschel