Buggy python submission (verdict: Runtime Error):
import collections # deque, Counter
# import itertools   # accumulate, compress, permutations(nPr), combinations(nCr)
# import bisect      # bisect_left(insert位置), bisect_right(slice用)
# import math        # factorical（階乗) # hypot(距離)
# import heapq
# from fractions import gcd # >=Python3.5 # lcm（最小公倍数） = (a*b)//gcd(a,b)
# from fractions import Fraction
# from math import gcd # <Python3.5
# --------------------------------------------------------------

class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]

    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)

        if x == y:
            return

        if self.parents[x] > self.parents[y]:
            x, y = y, x

        self.parents[x] += self.parents[y]
        self.parents[y] = x

    def size(self, x):
        return -self.parents[self.find(x)]

    def same(self, x, y):
        return self.find(x) == self.find(y)

    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    def group_count(self):
        return len(self.roots())

    def all_group_members(self):
        return {r: self.members(r) for r in self.roots()}

    def __str__(self):
        return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())


n,m = map(int,input().split())
lis = [list(map(int,input().split())) for _ in range(m)]
 
cmax  = 0
 
uf = UnionFind(n)

for i in lis:
    uf.union(i[0],i[1])

for i in range(n):
    cmax = max(cmax, uf.size(i))

print(cmax)

Fixed submission:
import collections # deque, Counter
# import itertools   # accumulate, compress, permutations(nPr), combinations(nCr)
# import bisect      # bisect_left(insert位置), bisect_right(slice用)
# import math        # factorical（階乗) # hypot(距離)
# import heapq
# from fractions import gcd # >=Python3.5 # lcm（最小公倍数） = (a*b)//gcd(a,b)
# from fractions import Fraction
# from math import gcd # <Python3.5
# --------------------------------------------------------------

class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]

    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)

        if x == y:
            return

        if self.parents[x] > self.parents[y]:
            x, y = y, x

        self.parents[x] += self.parents[y]
        self.parents[y] = x

    def size(self, x):
        return -self.parents[self.find(x)]

    def same(self, x, y):
        return self.find(x) == self.find(y)

    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    def group_count(self):
        return len(self.roots())

    def all_group_members(self):
        return {r: self.members(r) for r in self.roots()}

    def __str__(self):
        return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())


n,m = map(int,input().split())
lis = [list(map(int,input().split())) for _ in range(m)]

road = [[] for i in range(n)] # 頂点vと隣接する頂点のリスト
 
for i in lis:
    i[0]-=1
    i[1]-=1
    road[i[0]].append(i[1])
    road[i[1]].append(i[0])

cmax  = 0
 
uf = UnionFind(n)

for i in lis:
    uf.union(i[0],i[1])

for i in range(n):
    cmax = max(cmax, uf.size(i))

print(cmax)