Buggy python submission (verdict: Time Limit Exceeded):
import sys
import math
from collections import deque

sys.setrecursionlimit(1000000)
MOD = 10 ** 9 + 7
input = lambda: sys.stdin.readline().strip()
NI = lambda: int(input())
NMI = lambda: map(int, input().split())
NLI = lambda: list(NMI())
SI = lambda: input()


#隣接リスト 1-index
def make_adjlist_nond(n, edges):
    res = [[] for _ in range(n+1)]
    for edge in edges:
        res[edge[0]].append(edge[1])
        res[edge[1]].append(edge[0])
    return res


def main():
    N, M = NMI()
    F = {}
    for _ in range(M):
        a, b = NMI()
        F[(a, b)] = 1
    tree = make_adjlist_nond(N, list(F.keys()))

    ans = 0
    for start in range(1, N+1):
        que = deque()
        que.append(start)
        seen = [0] * (N+1)
        seen[start] = 1
        fri_cnt = 1
        while que:
            now = que.popleft()

            for goto in tree[now]:
                if seen[goto]:
                    continue
                seen[goto] = 1
                fri_cnt += 1
                que.append(goto)
        ans = max(fri_cnt, ans)
    print(ans)





if __name__ == "__main__":
    main()

Fixed submission:
import sys
import math
from collections import deque

sys.setrecursionlimit(1000000)
MOD = 10 ** 9 + 7
input = lambda: sys.stdin.readline().strip()
NI = lambda: int(input())
NMI = lambda: map(int, input().split())
NLI = lambda: list(NMI())
SI = lambda: input()


def make_grid(h, w, num): return [[int(num)] * w for _ in range(h)]


class UnionFind:
    def __init__(self, n):
        #親要素のノード番号を格納　xが根のとき-(サイズ)を格納
        self.par = [-1 for i in range(n)]
        self.n = n

    def find(self, x):
        #根ならその番号を返す
        if self.par[x] < 0:
            return x
        else:
            #親の親は親
            self.par[x] = self.find(self.par[x])
            return self.par[x]

    def is_same(self, x, y):
        #根が同じならTrue
        return self.find(x) == self.find(y)

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

        #木のサイズを比較し、小さいほうから大きいほうへつなぐ
        if self.par[x] > self.par[y]:
            x, y = y, x

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

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

    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.par) 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())


def main():
    N, M = NMI()
    F = [NLI() for _ in range(M)]
    uf = UnionFind(N)
    for a, b in F:
        a, b = a-1, b-1
        uf.unite(a, b)
    S = [uf.size(x) for x in range(N)]
    print(max(S))


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import sys
import math
from collections import deque

sys.setrecursionlimit(1000000)
MOD = 10 ** 9 + 7
input = lambda: sys.stdin.readline().strip()
NI = lambda: int(input())
NMI = lambda: map(int, input().split())
NLI = lambda: list(NMI())
SI = lambda: input()


#隣接リスト 1-index
def make_adjlist_nond(n, edges):
    res = [[] for _ in range(n+1)]
    for edge in edges:
        res[edge[0]].append(edge[1])
        res[edge[1]].append(edge[0])
    return res


def main():
    N, M = NMI()
    F = [NLI() for _ in range(M)]
    tree = make_adjlist_nond(N, F)
    
    ans = 0
    for start in range(1, N+1):
        que = deque()
        que.append(start)
        seen = [0] * (N+1)
        seen[start] = 1
        fri_cnt = 1
        while que:
            now = que.popleft()

            for goto in tree[now]:
                if seen[goto]:
                    continue
                seen[goto] = 1
                fri_cnt += 1
                que.append(goto)
        ans = max(fri_cnt, ans)
    print(ans)





if __name__ == "__main__":
    main()

Fixed submission:
import sys
import math
from collections import deque

sys.setrecursionlimit(1000000)
MOD = 10 ** 9 + 7
input = lambda: sys.stdin.readline().strip()
NI = lambda: int(input())
NMI = lambda: map(int, input().split())
NLI = lambda: list(NMI())
SI = lambda: input()


def make_grid(h, w, num): return [[int(num)] * w for _ in range(h)]


class UnionFind:
    def __init__(self, n):
        #親要素のノード番号を格納　xが根のとき-(サイズ)を格納
        self.par = [-1 for i in range(n)]
        self.n = n

    def find(self, x):
        #根ならその番号を返す
        if self.par[x] < 0:
            return x
        else:
            #親の親は親
            self.par[x] = self.find(self.par[x])
            return self.par[x]

    def is_same(self, x, y):
        #根が同じならTrue
        return self.find(x) == self.find(y)

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

        #木のサイズを比較し、小さいほうから大きいほうへつなぐ
        if self.par[x] > self.par[y]:
            x, y = y, x

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

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

    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.par) 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())


def main():
    N, M = NMI()
    F = [NLI() for _ in range(M)]
    uf = UnionFind(N)
    for a, b in F:
        a, b = a-1, b-1
        uf.unite(a, b)
    S = [uf.size(x) for x in range(N)]
    print(max(S))


if __name__ == "__main__":
    main()