items = ['one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine', 'ten', 'eleven', 'twelve', 'tree']

count = 4
mustContain = ["e", "i"]  # all of these characters at least once
mustNotContain = ["n", "o"]  # none of those chars

hits = [
    item for item in items if
    len(item) == count and
    all([c in item for c in mustContain]) and
    all([c not in item for c in mustNotContain])
]
print(hits)