There is no specific diet that can increase the concentration of sperm in an ejaculate, although there are, of course, some poor diets that can reduce it. Alcohol and smoking are obvious things that can reduce it, although I have to say that even these have a minimal effect until you reach alcoholic proportions or smoke over 20 cigarettes per day. A number of vitamins and minerals have been suggested as important for sperm production, such as zinc, selenium and calcium, but it would be sensible for a deficiency of any of these to be diagnosed before adding them to a diet.