There isn't a maximum health cap. The health number in your HUD will only show up to 255 (or 250, don't remember) and then reset once it passes that, but your actual health value will differ from what the HUD displays.
If you still wanted to do this, you'd do something similar to:
Code:
#define MAX_HEALTH 255
#define ADD_HEALTH 100
set_user_health( id, min( ( get_user_health( id ) + ADD_HEALTH ), MAX_HEALTH ) )
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