LEGO sets usually have a number of extra pieces. In most cases they are small bricks, that are easier to lose like 1x1 plates/tiles/studs, cheese slopes, levers/antennas and flower petals.

Does LEGO send free replacement parts?

For the vast majority of bricks, the replacement will be completely free, and LEGO won't even ask for proof of purchase. Typically, replacement pieces take 7-10 business days (depending on location) to show up at your door—LEGO also covers the shipping cost.

Do LEGO piece counts include extra pieces?

No, excluding the extra pieces, 30476 (pictured below) has exactly the 33 plastic pieces the manual says it should have.