That is the #37, close to gate 18 and at the same floor of shops and restaurants.
Lounge #21 is downstairs.
If you have never been there, then go to gate #21.
If it is manned then show your lounge pass and they let you go downstairs to the lounge.
Otherwise just go, the gate is open (usually).
As MANman wrote don't expect a designer concept lounge, showers, pool tables or panoramic views, but just a place were you can sit a bit more comfortably.
The food is ok, as well as the choice of snacks and drinks.
Maybe you may get access to #20 which is better, if you ask politely they may let you go in there.

Terninal 2 is much better (and the CA lounge deserves a visit) and all lounges are better placed than T1.
But then, between the two terminals there are also 9 years difference and this may explain this bizarre situation.

SHA is fine as long as we consider T2 (domestic).
T1 (international) is a joke by today's standards, but then again T2 was opened in 2010 while the last big renovation at T1 took place during the end of the '80s.

再见
Ulxima