Woflje's Level Up HP datapack makes you start with 5 hearts (10 HP) which you can increase to a max of 20 hearts (40 HP) by killing mobs! The more hearts you have, the more mobs you have to kill for a level up.
This datapack was made to resemble the Level Up HP! mod.
To customize any values such as the starting HP, the max HP or the required kills per level up, you can change the parameters at the top of the 'compiler.py' file to your likings and run the python file. This will modify the files in the data folder, and update the datapack file.