I’m not sure where it began, but someone started the tale that the Hebrew High Priest had a rope tied to his leg, so that if the sound of the bells attached to his robe stopped jingling in the Most Holy Place, the people on the other end of the rope would know he’d been struck dead and could haul him out. No one exactly knows who began this myth: it’s not in the Bible nor is it in the Talmud. Yet you’ll find it trotted out in sermons fairly regularly.
Another of those legends is the idea that there are 613 commandments in the Mosaic Law. In this case, we do know the origin of the legend: rabbinic Judaism. In fact, the number 613 was obtained through typical rabbinical logic. One way is that the numerical value of the Hebrew word Torah is 611, to which can be added the first two of the Ten Commandments. Another method was to say that there were 365 prohibitions – corresponding to the days in a year – and 248 commands – corresponding to the bones in the human body. But in fact, to obtain this number, commands or prohibitions are often counted twice, while others are subsumed. The number 613 was first suggested by a third century rabbi and mostly codified by Maimonides in the 11th century. Virtually all the commandments that Maimonides lists as commands are not explicitly in the Torah, but are rabbinic interpretations of the Torah. A later rabbi, Gersonides, counted 513. Another suggested 521. Eliezer ben Samuel listed 417 commandments. The number of 613 wasn’t really accepted until much later in rabbinic Judaism’s history.
However, you’ll find Christian teachers readily throwing out the number 613, as if it is as biblically codified as the Ten Commandments (Ex. 34:28, Deut. 4:13). I’m sure I used the number myself, simply assuming that someone else counted correctly.
In fact, I haven’t come across a Christian teacher who has meticulously enumerated the commandments in the Law (without simply hijacking or modifying Maimonides’ list). And no wonder: it would be an elusive goal. You’d have to decide whether to count repeated commands in Deuteronomy twice. You’d have to determine if a sentence that commands three actions counts as one command or three. You’d have to determine whether a ritual command contains a moral imperative. And if you made the effort, you would almost certainly not land on the number 613, unless you had set it as a target before beginning.
Yes, it’s easier to take the rabbis’ word for it. But since they are often wrong about a great many things, I’d suggest saying “all the commandments of the Law” or “the entire Torah” or “every law of Moses”.