How many 2x4's will you need to build a wall 16ft long and 8ft high?

The material list is:

2 - 2 x 4 x 16

13 - 2 x 4 x 8

The first 16 footer serves as the base plate.

The second 16 footer serves as the top plate.

The thirteen 8 footers are the studs which comprise the structural substance, if this wall calls for them to be spaced 16 inches apart which is the most common spacing. Since you didn't specify the spacing, this is assumed.

The spacing of the final stud will only be 12.5" from its neighbor. This closer spacing gives an overall length of exactly 16 feet. If the prior 16" oc spacing is continued, the wall will be 16'-3.5" long