This is assuming Xie Xun is around the same level as the other Ming cult guardians.
The 20 people that I have in mind for >= Xie Xun are:
1. Zhang Wuji
2. Zhang Sanfeng
3. Yang Dingtian
4. Lady Yang
5. Xuanming Elder Lu Zhangke
6. Xuanming elder He Biweng
7. Du Wei
8. Du Nan
9. Du Jie
10. Kongjian
11. Cheng Kun
12. Hao Mi
13. Bu Tai
14. Kongwen
15. Kongzhi
16. Kongxing
17. He Zhudao
18. Eight Armed Swordsman Fang Dongbai
19. Ah Er
20. Ah San
And if we are to continue the list, we can include Yang Xiao, Fan Yao, some of the 7 Heroes of Wudang, Zhuang Zheng, Miejue, and the other 3 Guardians.
So actually, I think there are about
30 people in HSDS that better than or equal to Xie Xun.