What is "all the mitzvah"

(1) [The meaning is] in accordance with the plain sense. And the midrash is: If you have begun a commandment, be sure to finish it, for only the one who finishes it get the credit, as it is written (Joshua 24:32), "The children of Israel buried the bones of Joseph, which they had brought up from Egyp, in Sh'khem." But did not Moses alone bring the bones up (from Egypt)? Rather, (this teaches us that) because he did not manage to finish the commandment (of burying Joseph), the children of Israel did it and they received the credit.

The meaning is: If you want to observe the mitzvot so that you will live, remember the way.

That is to say, even though I warned you with rules, still do all of the mitzvot. Because when you do all of them, you will live and multiply with offspring and livestock and produce, as is mentioned above.

It began in the singular and finished in the plural, since the Righteous One is the Foundation of the Universe. And even an individual who did one mitzvah, happy is he since he moved himself and the entire world to the pan of merit, etc. So too an individual who did teshuvah, he is forgiven as well as the whole world. Thus it says "which I command you -singular- be sure (pl) to do them so you (pl) should live," because the observance of the individual is as if the multitude observed the commandment, and the benefit reaches everybody.

And it described the mitzvah in the singular to tell you that even observing one mitzvah fittingly causes "that you should live", like the opinion of Rav Yehuda in Sanhedrin (111) who explained the verse "Her mouth opened lawlessly" (Is 5:14) to refer to one who didn't observe even one commandment. But if he observed even one commandment he is saved from Hell, since one commandment leads to another, as is explained in Parshat Shlach Lecha (15:38).