Here is another way to do it in Python:

my_string = input('Enter your string:')
print(my_string.translate({i+48:57 for i in range(9)}).count('9'))

Basically, you convert all the digits from 0 to 8 in you string to nines and then count the nines. The small dictionary is just here to convert each digit from '0' (character number 48) to '8' (character number 56) to a '9' (character number 57). You can create this dictionary using the maketrans method :

tab = maketrans('012345678', '9'*9)
my_string = input('Enter your string:')
print(my_string.translate(tab).count('9'))

Here is another way to do it, using the isnumeric method, as well as map and sum.

my_string = input('Enter your string:')
print(sum(map(lambda x:x.isnumeric(), my_string)))

map applies the function lambda x:x.isnumeric() -whose purpose is to take a string and returns True if x is a number- to each character of my_string.
sum adds all the True and False, considering them as 1s and 0s.

Finally, here is another way to do it using the powerful re module:

import re
my_string = input('Enter your string:')
print(len(re.sub('[^0-9]', '', my_string)))

The regular expression '[^0-9]' means every character but a digit. All these characters are replaced by the empty string '' (or "" if you prefer).

In what language do you want to do this? In Python you can use something like

string = "abcdef54w6ed7fuyigy86giug75r54f789yu9ou2j89wrhg" # random string
counter = 0
for char in string:
  if char == '0' or char == '1' or char == '2' or char == '3' or char == '4' or char == '5' or char == '6' or char == '7' or char == '8' or char == '9':
    counter += 1
print(counter)