How about just checking the stamina value?
const MAX_STAMINA = 100
func _recharge(stamina):
#your code
if stamina > MAX_STAMINA:
stamina = MAX_STAMINA
UPDATE:
I wanted to add another example since Wakatta had a brilliant idea to use clamp (see in the comments).
const MIN_STAMINA = 0
const MAX_STAMINA = 100
var stamina = 79
func _ready():
_recharge(20)
_recharge(20)
func _recharge(stamina_increase):
stamina = clamp(stamina + stamina_increase, MIN_STAMINA, MAX_STAMINA)
print(stamina)
This will give you the following output:
99
100