Addon: Double HP
Double HP is a lightweight health addon that raises the player's capacity from the vanilla 10 hearts to 20 or 30 hearts. It is designed for players who want more room for mistakes without replacing the rest of the survival progression.
Author: Ronix_Studios

Extra health makes long exploration, battles against large groups of mobs and difficult challenges more forgiving. It can also change PvP pacing because every player takes longer to defeat.
The current 1.2 release includes a redesigned HP Stick model and fixes the known issues from the previous beta version. Small problems may still remain, but this is the corrected full package rather than the older beta download.
How to install the add-on
Download the .mcaddon or .mcpack file, open it with Minecraft and wait for the import to finish. Then enable the resource and behavior packs in the world settings. If the add-on requires experiments, turn them on before launching the world.