limit_to_10.py

import csv
from collections import defaultdict

lines = []
total_values_per_group = defaultdict(int)

with open('/Users/vicente.rocha/Downloads/david.txt') as f:
    csv_reader = csv.reader(f, delimiter=',')
    # iterate and add only 20 values per group
    for row in csv_reader:
        index = row[0]
        value = row[1]
        total_values_per_group[value] += 1
        if total_values_per_group[value] <= 20:
            lines.append((row[0], row[1]))

# verify
validator = defaultdict(int)
for l in lines:
    validator[l] += 1
for val, size in validator.items():
    if size != 20:
        print(f"{val} has size {size}")

# write to disk
with open('/Users/vicente.rocha/Downloads/david_fix.txt', 'w') as f:
    for l in lines:
        f.write(f"{l[0]},{l[1]}\n")