It is dependent on the user
Using 25% of its maximum HP, the user creates a substitute with 1 HP more than the amount of HP lost by the user. If the user's maximum HP is 3 or less, it will not lose any HP when the substitute is made. A substitute will have the same current type and current stats as the Pokémon that created it.
So, if the Pokemon had 100 HP, the substitute will have 26 HP.
The higher the HP of the Pokemon, the higher HP the substitute will have.