Please see the sample code below.

=========================== sample code ===========================
#include <iostream>
#include <unordered_set>

template<typename S>
void print(const S& s, typename S::const_iterator& it)
{
  std::cout << "size = " << s.size() << '\n';
  std::cout << "bucket_count = " << s.bucket_count() << '\n';
  std::cout << "load_factor = " << s.load_factor() << '\n';
  std::cout << "max_load_factor = " << s.max_load_factor() << '\n';
  std::cout << "size limit = " << s.bucket_count() * s.max_load_factor() << "\n\n";

  for (std::size_t i = 0; it != s.end() && i < s.size() / 2; ++i, ++it) {
    std::cout << *it << ' ';
  }
  std::cout << "\n\n";
}

int main()
{
  std::unordered_set<int> s;
  const auto max = 10;

  s.reserve(max);
  for (int i = 0; i < max; ++i) {
    s.insert(i);
  }
  auto it = s.cbegin();
  print(s, it);

  s.insert(max);
  print(s, it);
}
=========================== sample code ===========================
=========================== output ===========================
size = 10
bucket_count = 11
load_factor = 0.909091
max_load_factor = 1
size limit = 11

9 8 7 6 5 

size = 11
bucket_count = 23
load_factor = 0.478261
max_load_factor = 1
size limit = 23

4 5 6 7 8 
=========================== output ===========================

cf. https://wandbox.org/permlink/QhPfL6787GV8RYXV

The C++17 standard 26.2.7[unord.req]/p.15 says,

  The insert and emplace members shall not affect the validity of iterators if
  (N+n) <= z * B, where N is the number of elements in the container prior to the
  insert operation, n is the number of elements inserted, B is the container's
  bucket count, and z is the container’s maximum load factor.

So, I think that the sample code above should never rehash.