Which side of Jesus' body was pierced?
The short answer
John tells us Jesus' side was pierced but not which side. Crucifixes are artistic, not exact, so showing the wound on the right may be symbolic: in Scripture those on Jesus' right are the saved, and they are saved by his blood.
What John says
John records that Jesus' side was pierced (John 19:32-35) but does not say which side.
Art and symbol
Crucifixes are artistic renderings, not exact reproductions. Placing the wound on the right may be symbolic: those on Jesus' right are the ones to be saved (Matthew 25:32-33), and they are saved by the blood he shed for them (Matthew 26:28).