from itertools import combinations

my_list = ["python", "is", "fun", "to", "learn"]

print("The list is :")
print(my_list)

my_target_str = "onis"

my_result = -1
my_set_string = set(my_target_str)
complete_val = False
for value in range(0, len(my_list) + 1):

   for sub in combinations(my_list, value):

      temp_set = set(ele for subl in sub for ele in subl)

      if my_set_string.issubset(temp_set):
         my_result = value
         complete_val = True
         break
   if complete_val:
      break

print("The result is :")
print(my_result)

The list is :
['python', 'is', 'fun', 'to', 'learn']
The result is :
2