The 140th mitzvah is that we are commanded to count the years, beginning with the time we ruled over Eretz Yisrael and took possession of it.1
This mitzvah of counting the shemittah years is to be fulfilled by the beth din, i.e., the Great Sanhedrin. They have the responsibility of counting each of the 50 years, just as each of us is required to count the days of the omer.
The source of this commandment is G‑d's statement,2 "You shall count seven Sabbatical years."
The Sifra says: "One might think that they count seven consecutive Sabbatical years and then make a jubilee year. Therefore the verse continues, 'seven years, seven times.' If not for these two phrases, we would not know the law."
This means that without both phrases we would not understand how to perform this mitzvah, i.e., to count just the years and to count the appropriate shemittah cycle together with it.
Their3 statement that the law could not be derived without both passages definitely implies that they comprise one mitzvah. If they were two mitzvos — one to count the years and one to count the shemittah cycles — our Sages would not have used the expression "If not for these two phrases, we would not know the law"; because any two mitzvos are derived from two verses — each from its own verse. One can only say, "If not for these two phrases, we would not know the law" regarding one single mitzvah the laws of which can only be fully understood through two phrases.
An example of this is the b'chor (first-born animal). One verse,4 "The first-born initiating every womb is Mine" implies that both males and females are included. Another verse,5 "[every animal that your cattle shall issue] which is male shall be the L‑rd's" implies both first-born animals and those which are born later. Only with both verses do we fully understand the mitzvah — that it applies only to first-born male animals, as explained in the Mechilta.
