I mean, it depends on how much weight you want to add.

According to the internet, a penny weighs 2.5g or 0.088oz. A roll of pennies costs 50c and adds 125g or 4.4oz.

Some prints I've added just a few pennies to, and some a few rolls. It just depends what you need.

Well, this is how you horde them for when they become novelties at some point in the future.