import random

a = [1, 2, 3]
b = ['a', 'b', 'c',  'd', 'e']
c = []
d = list(range(1, 101))
num_counter = letter_counter = 0
loop_counter = 100

for i in range(loop_counter):
    if random.choice(d) >= 21:
        c.append(random.choice(a))
        num_counter += 1
    else:
        c.append(random.choice(b))
        letter_counter += 1

print('n', c)
print('Numbers: ', num_counter)
print('Letters: ', letter_counter)

from string import ascii_letters, digits
import random

total = 100
nletters = round(0.8 * total)
ndigits = total - nletters
print(nletters, ndigits)  # -> 80 20

result = [random.choice(ascii_letters) for _ in range(nletters)]
result += [random.choice(digits) for _ in range(ndigits)]

result = [random.choice(ascii_letters if random.random() < 0.8 else digits)
          for _ in range(total)]

>>> len(list(filter(str.isdecimal, result)))
23