Do bosses health scale in Terraria?
Only health scales, not damage. It is per server, so anyone logged in to that server contributes to boss health scaling. This means having more players can potentially be detrimental if they are not actively contributing and carrying their weight towards boss damage.
Do more players increase boss health Terraria?
Bosses in Expert Mode have increased health. Their health is boosted even further in multiplayer, with more health added for each player.