The conversion rate of your webpage would be calculated by dividing the number of people who bought something by the total number of people who viewed your webpage, and then multiplying by 100 to get a percentage. Using the numbers you provided, the conversion rate would be:
100 (purchases) / 500 (views) x 100 = 20%
Calculating the conversion rate is important because it gives you an idea of how effective your webpage is at converting visitors into customers.