violentlie

students = [
    {'first_name': 'Вася'},
    {'first_name': 'Петя'},
    {'first_name': 'Маша'},
    {'first_name': 'Маша'},
    {'first_name': 'Петя'},
]

names = dict()

for student in students:
    if student['first_name'] not in names.keys():
        names[student['first_name']] = 1
    else:
        names[student['first_name']] += 1

for name, count in names.items():
    print(f'{name}: {count}')

names = [s['first_name'] for s in students]

for name in set(names):
    print(f'{name}: {names.count(name)}')

from collections import Counter
from operator import itemgetter

Counter(map(itemgetter('first_name'), students))

Counter(map(lambda student: student['first_name'], students))