Is there an effect of mountain dew on sperm count?

Probably not: There is some speculation whether the yellow dye in mountain dew will affect sperm parameters. The caffeine in mountain dew actually has no effect. A good rule of thumb is that things should be done in moderation.

Uncertain: In one Danish Study men who drank more than one liter of soda per day did have reduced sperm count. However, those men also had less fruit / vegetable intake and higher fast foods intake. So it isn't known if the soda was causative. I don't know which sodas the men drank. http://www.dailymail.co.uk/health/article-1262759/Drinking-cola-lower-mens-sperm-count.html