new = ['coin', 'coin', 'apple', 'axe', 'sword']
items = {'coin': 43, 'apple': 5}

for x in new:
    if x in items:
        items[x] += 1
    else:
        items[x] = 0

print(items)

#{'sword': 0, 'coin': 45, 'apple': 6, 'axe': 0}

items = {'coin': 43, 'apple': 5}
new = ['coin', 'coin', 'apple', 'axe', 'sword']

from collections import Counter
def add_to_dict(x: dict, y: list):
    for k, v in Counter(y).items():
        x[k] = x.get(k, 0) + v
add_to_dict(items, new)
print(items)

>>> from collections import Counter
>>> Counter(items) + Counter(new)
Counter({'coin': 45, 'apple': 6, 'axe': 1, 'sword': 1})