This is determined based on your card collection. Basically, collecting more cards and upgrading them will increase your Player Level.
Your Base Health depends on your level, the higher your Player Level is the higher your Base Health will be. There will be though some levels where your Base Health will not increase. Listed below is guide.
Level 1-3:
Base Health 10
Level 4-6:
Base Health 11
Level 7-8:
Base Health 12
Level 9-10:
Base Health 13
Level 11-12:
Base Health 14
Level 13-14:
Base Health 15
Level 15-16:
Base Health 16
Level 17:
Base Health 17
Level 18:
Base Health 18
Level 19:
Base Health 19
Level 20:
Base Health 20