It is kind a silly question (I know), but where can I find after how many sales a product becomes a Bestseller?
Depends on over setting.. Using time period we can set how many days ordered best selling products. Ex: If I set 5 in the time period then it will display best selling products which will be sold in the last 5 days. If no value set then it will display last 60 days best selling products.
Also go through the link.. https://www.magentocommerce.com/magento-connect/best-seller-products-extension.html