number - Why not count 50 days of the omer instead of 49?

The verse in Vayikra (23:16) says:

עד ממחרת השבת השביעית תספרו חמישים יום
even unto the morrow after the seventh week shall ye number fifty days (OJPS, emphasis added)

In my experience the omer is only counted for 49 days. Is that accurate? In light of the above verse, why is that so?

Answer

For a start, the words before the ones you mention are:

שֶׁבַע שַׁבָּתוֹת, תְּמִימֹת תִּהְיֶינָה

Seven complete weeks are only 49 days.

Secondly, we see other times in Chumash that a number means "until, but not including" that number. For example in Devarim (25:3):

אַרְבָּעִים יַכֶּנּוּ, לֹא יֹסִיף

We only strike him 39 lashes; all the up to - but not including - 40.

That's also the simple explanation for the 70 souls that came down to Egypt with Yaakov: Breishith (46:27)

כָּל-הַנֶּפֶשׁ לְבֵית-יַעֲקֹב הַבָּאָה מִצְרַיְמָה, שִׁבְעִים

If you count them you only get to 69; all the up to - but not including - 70. (And yes, there's a known Medrash that the 70th was Yocheved born as they came through the gates.)