How many people have viewed or bought your product in a certain timeframe - counter.
Social proof is a powerful marketing concept that can increase sales. It basically shows people that your product is popular. This can give visitors trust to buy your product too.
I think the best way is to but such a counter directly next to the product or buy button.
Below some screenshots of companies already using this concept too.
Amazon - Number of sales last month
Ebay - Watched and sales counter
