About how many #8 x 2 1/5 exterior screws come in a 5lb. box

about 400

Of course, they don't tell you the count per pound, but based on an unnamed large Internet search engine search for "screws per pound by size" I found a handy chart (which I'm prohibited from linking to per the Lowe's Terms and Conditions) that says that a screw of this size would have approximately 110 screws per pound. (They didn't list this exact size, but using the amazing power of Math I could estimate between sizes and confirm by searching other screw sites.) 5 pounds would then have about 500-600 screws.